In The North Remembers, Sansa Stark pretty much takes a level and then some. She first kills Ser Shadrich with a knife when he attempts to rape her, then manages to enlist both the Brotherhood after convincing them not to kill Sandor, and the Burned Men by willingly getting her hand burnt, to help her overthrow Littlefinger. This finally culminates with them assaulting the Eyrie whilst Sansa is being tried for murdering Sweetrobin Arryn. The end result, Sansa beating Littlefinger at his own game (along with him getting a well-deserved beatdown courtesy of Lady Stoneheart).
In Redeemers Assemble, multiple characters do this. Ultron rips his old body in half with his new one to threaten Attuma, flies headfirst into an orbital strike, and almost dies to boot, Abomination lifts the city of New York to stop it from sinking, and Red Skull sends a wave through a portal INTO THE SUN
Calvin started out as the Idiot Hero who was as immature and inexperienced as his comic strip counterpart. Come at least three seasons later, he is a more cunning, kinder, and is actually able to handle himself perfectly fine with handling extremely dangerous villains by himself by the Season 4 finale "Our Solemn Hour".
Children of an Elder God: The three pilots take levels constantly: every time they kill an Eldritch Abomination they steal its power, and their bodies evolve to adapt to their enemies. So, every battle they grow more powerful.
At the beginning Shinji is his shy, wimpy canon self. Then he discovers that his body got modified with Kryptonian DNA and becomes Superman.
Asuka also took several levels when she claimed her Amazonian heritage or -depending on the version- her body got modified with Kryptonian DNA. Suffice it say, she did not got upset when she was unable to synch with her giant robot because she "can fly and bench press a cement mixer".
In the original version several more characters became versions of several superheroes of The DCU, ergo, more badass (for example, Touji became a Green Lantern).
In the Chance Encounter series, Paris, Helen and Briseis, the former becoming a skilled archer, the latter two being skilled swordswomen, and Balian takes a subtle one, picking up a bunch of new moves from the Fellowship and moving more smoothly in battle, going from ordinary Badass Normal levels to, while under mind control and what could be considered a Superpowered Evil Side, fighting Legolas, who is considered to be the hallmark of badassery in the series, to a standstill.
In Child of the Storm, everyone seems to have taken one. Everyone is closer to comics power levels, Thor and Loki most noticeably, Clint only has the real life Legolas to compete with him for the title of best shot in the Nine Realms and Tony is stated to have curbstomped Volstagg in an offscreen sparring match (mitigating factor: Volstagg held back because he wasn't sure how tough Tony was in his armour) and Fury is stated to be the reason Lucius Malfoy walks with a limp and a walking stick.
The Winter Soldier, already a badass, has become The Dreaded, a terrifying One-Man Army. Previous feats include one shotting a former Captain Britain and mowing through two squads of Europe's finest Aurors. On screen, he's killed a Nundu (takes 100 Wizards to subdue) and a Chimera without taking a scratch or even slowing down, mows through a number of extremely well trained soldiers, including a two second take down of James fucking Bond, tears through MI13, takes on Black Widow, Hawkeye and Captain America at once and wins, taking the first two out with a booby trap and ends up almost beating Cap to death with his own shield, kills an enhanced werewolf the size of a car with his bare hands, then carrying Carol Danvers down a treacherous mountain in a ferocious storm before she got frostbite. He justly earns both Loki's assessment as possibly the deadliest assassin in the Nine Realms and the comparisons to immortal assassin, Jared 'The Hellhound' Kincaid.
In chapter 70, he takes on, in succession, a couple of dozen or so Aurors and Ministry personnel in an assault on the Ministry itself, a couple of dozen SHIELD Agents on the Helicarrier, Captain America, Black Widow and Hawkeye at the same time on the now sinking Helicarrier, multiple Secret Service Agents in an assault on the White House, and Wolverine. And he does this while holding back to spare as many people as possible.
As of Chapter 50, he's removed Von Strucker, taken control of the Winter Soldier, and with him, HYDRA. Which turns out to have been what he was planning to do all along. By chapter 74, he's a bona fide Evil Overlord having neutralised the Avengers, crippled SHIELD, stolen multiple tons of Vibranium and sauntered into the UN to extract the surrender of the world, backed up by a now Vibranium hulled teleporting Helicarrier. While the backlash the following chapter is epic, there's no denying that he's badass.
All of the kids take a level or several dozen in/by chapters 59 and 60. Harry is, by this point, capable of sending a moderately powerful Winter Fae running for its life, before blasting it out of the sky. Carol is capable of coming up with an on the fly plan which Uhtred comments would impress Sif. And then, after the temporary Plot-Relevant Age-Up, Harry's pummelling supersized werewolves to mush, burning holes in mountains, flying, punking Project Ultimatum, a proto Destroyer that flies from the Colorado Rockies to well East of Moscow and impressing Baron Zemo, who compliments him, Diana's tearing Project Ultimatum apart with her bare hands while Zemo is trying to use it to incinerate her, Jean-Paul's showing how deadly rocks thrown at mach speeds can be, Uhtred's treating a giant werewolf trying to eat him as an inconvenience - he's mainly annoyed that no one else is paying attention, Carol's shown such potential as a strategist that the fucking Winter Soldier notices, and his analytical Soldier personality suggests that she is the main threat, as she's holding the others together as a group, and must be eliminated. While the Bucky side of the Soldier objects, violently it's a real mark of how badass you are that, on a battlefield with three PhysicalGods, two of whom possess raw strength/power in the Thor range, one of which has a lot of combat training, the other of which has some and is excellent at improvising, the third of which is the protégé of Sif herself with a very big axe, and a potentially very dangerous speedster, with only basic peak human/Super Soldier abilities, you're considered the main threat by the Winter Soldier himself.
By chapter 70, Harry can mow through a HYDRA assault force before getting temporarily killed by Daken. Having learnt his lesson, by chapter 74 he can stop bullets in midair, hold a HYDRA assault force at gunpoint with their own weapons and deliver a bitter "World of Cardboard" Speech without missing a beat, and by chapter 75 he flattens Daken in their rematch and the only thing that causes him problems is Gravemoss. Also, Jean-Paul's a more ruthless version of the Flash and Carol becomes the Green Lantern.
The Naruto/Justice League Crossover Connecting The Dots does this to most of the Konoha 11, but especially to Tenten. You remember Tenten? Trust me. Not this one.
The Simpsons/Futurama crossover Fast Forward has pretty much all the main characters of either show take major levels in badass (with the exception of Leela, given she was pretty badass to begin with), due to Bender gaining a weaponry upgrade and the human characters (and Zoidberg) gaining a Farnsworth-made upgrade: their own customized, stylish, Transformers-style suit of Powered Armour.
In the Magical Girl Lyrical Nanoha and Harry Potter crossover Professor Takamachi, Nanoha manages to do this with a lot of Hogwarts students through a simple means. Physical training to increase physical and magical stamina, math to increase their ability to focus and multitask better, supervise the DA by putting things into practical terms, and then organizing groups to do magical sparring together as teams in simulated combat. This pays off when the Hogwarts students organize and decimate a Death Eater attack on Hogwarts. It is also hinted that Harry and his group put up a much better fight in the Department of Mysteries.
While in The Wizard in the Shadows, Harry has mostly done his levelling offscreen by the time the story starts, it is quite something to contrast the already badass Boy-Who-Lived to what he is after four years trapped in Middle Earth in the run up to the War of the Ring: an absurdly - by human standards - fast Knife Nut, an extremely powerful wizard who is more than willing to kill and capable of going toe to toe with the Nazgul - he actively hunts them, and The Dreaded for everything evil in Middle Earth for very good reason, whispered of as 'The Black Wizard'. As noted by the author, this is largely justified by the Kill or be Killed nature of late Third Age Middle-Earth.
He takes further levels in badass, wielding the Sword of Gryffindor in battle, fending off an army of Dementors (admittedly, he was possessed for that part) after singlehandedly holding an Entropy Ward over all of Helm's Deep for over two hours against continuous assaults from the Uruk-Hai, then destroys a fifth of them by taking command of the storm and raining down lightning. And he was only drawing on outside power to a great extent with the Dementor part. He later takes on Wraith Voldemort, who is present as a balance to the Tenth Walker in a swordfight on the back of a fell beast. Among other acts of obscene badassery.
Just about every character in the series: Boromir and Aragorn each singlehandedly take down one of Sauron's Olog-Hai (uber-trolls) without taking a scratch. Ginny unleashes hell on the Morannon in the form of three firestorm tornadoes (with outside assistance). Ron and Hermione develop into capable sword fighters. Emrys goes from Punch Clock VillainMook who, after a Heel–Face Turn (debatable since he wasn't at all personally evil in the first place) becomes a Badass Normal who kills a (distracted) troll solo, and eventually becomes - at the Black Gate - a part Ainur Empowered Badass Normal who swats trolls like flies. By the sequel, it is vaguely hinted that he can fly, or at least defy gravity, and can move in Super Speed.
The Dazzlings in LEGO Equestria Girls 2 (even Sonata Dusk in a way), in part from the fact that the story does not take place in a high school. Sunset Shimmer also counts.
Hiccup is made into a Jedi Knight, having hid away all of these years since the order fell. While he has not tamed a dragon like his canon counterpart, Hiccup is one of the most skilled fighters in the series, having created his own Jedi martial art called "The Book of Dragons", basing every kick-ass technique off of a different dragon that dwelled on his home planet.
Elsa is made into a Jedi Knight, both as a competent fighter and master of what is called "Force Ice", a parallel to the dark side's Force Lightning and her ice magic in the film.
Anna is made into a pretend Mandalorian searching for her big sister. Not convinced that the Order betrayed the public like everyone else, Anna set out to find her. Along the way, she acquired a Mandalorian outfit, a gun and the skills to use it (if not a little ''too'' well, to be completely honest). To top it all off, Elsa says that she was able to find her by using the force unconsciously, but turns down her offer of Jedi training, happy with her own set of skills as is.
Despite being a padawan, Rapunzel is an incredibly competent fighter and is fluent in the Force, being able to use it to heal injuries.
Not only does Dagur retain all of the deranged, battle-crazy demeanor and skills from his canon incarnation, but he is now a god-damn Sith Lord named "Darth Skrill" with mastery over the force, is handy with a light-saber and the amoral temperament to threaten the entire universe with it.
If Drago Bludvist was not scary enough, now he is now a Sith Lord and the Emperor, ruler of the Empire and conqueror of planets.
In Wonderful, thanks to Taylor's suits increasing strength, speed... many characters become badasses (Emma goes from being an average girl to smashing a tank with a mop) or increase their badassery (Sophia).
Thousand Shinji: In the original series, the main characters were humans with average abilities. Here, Shinji becomes a sorcerer with psychic powers and fulgurkinesis, Asuka a Super Soldier and Rei a Plaguemaster.
After her suit is destroyed and she's gravely injured in Silly but Killy, Tali is given Limm Goo and Fightin Juice to possibly save her life. As a result, she becomes part Ork.
Keena Soga, of course, in Keena & the Defendants of Constan Magic Academy, who now no longer is forced to stay out of the way in the battle, takes tons of hits, still gets back up and doesn't even need help to defeat her opponents, even if she hurts them in quite brutal ways.
After the events of Omni-Tamer, Takato becomes the strongest Tamer by far. Unfortunately for him, the one person whose respect and praise he truly desires instead becomes aggressive and spiteful to him out of jealousy, forcing Takato to finally snap.
Dragon Age: The Crown of Thorns has given levels in badass to more than just the dwarven noble Wise Prince protagonist, plus some antagonists (the Sloth Demon, Piotin Aeducan, the Fade Beast-it becomes a rock wraith, etc.)
Kallian Tabris, the city elf, also a Lovable Rogue, finds herself forced to drink Avernus' alchemical potion, making her blood poisonous to darkspawn (and she can even read their attack intentions like some sort of battle precognition). Lady of War indeed.
Alim started out as a One-Man Army blood mage but eventually learned shapeshifting and telepathy. He also seems to avoid the Squishy Wizard trope altogether, though mostly through timely applications of forcefields. One can only wonder what will happen if he ever becomes an Arcane Warrior too.
Alistair didn't get many special powers, but Raonar's tutoring did yield a much higher level of self-confidence, actually making him lose a level in suck. A Magic Knight in his own right.
Theron Mahariel and Gwenith Cousland can still qualify as Badass Normal members, but it is unlikely that the story will end without them also getting some sort of powerup. One might say it is required, considering what happened in the latest chapters (The Archdemon became smarter and more self-aware, possibly able and mentally coherent enough to use blood magic).
Even Trian Aeducan seems to have gone through this after his memorable Break the Haughty several months-long period of being presumed dead after having his death faked against his will (and forced into being roomies with Gorim.
Tohru Honda — no, really— TohruHonda gets this treatment in the Western AU fic The Swordsman and the Gunslinger. On the surface, she appears to be a sweet but she can hold her own in a fight. Even a person who knows her very well is amazed that she can handle an enemy.
Dumbledore's Army and the Year of Darkness is all about Neville Longbottom, and the DA, levelling up in badass to the point where they're able to stand up to, and fight, Death Eaters on a nearly-equal-to-equal basis.
Another example is "In Cold Blood", wherein Harry is asked by Gringotts to work as a freelancer assassinating Death Eaters for them. The goblins have a magical method that allows them to transplant knowledge from prisoners, so that Harry can fight well beyond the level of his targets. The fic is rather vague about how much knowledge of what Harry gains: all it states is that the first donor is a skilled enough torturer to keep a man on the edge of death for six months, and that after Harry gains the torturer's skill a "seemingly endless" number of donors file in after him, with "everything Harry could possibly use" against his victims.
In "Harry gets Motivated", Harry asks Professor Moody to mind control him into relentlessly forcing himself to study to exclusion of everything except whatever Harry has to do to stay mentally and physically healthy, and to protect himself from Voldemort. This fic ends with Harry draining his bank account to further his studies, refilling it by attacking a giant vault in Malfoy Manor with his new skills, and eventually killing Voldemort so as to ensure he can study uninterrupted.
In Rise of the Galeforces, this happens to virtually ALL of the Supers who have been bought Back from the Dead at some point or another. Stratogale, Splashdown, and Thunderhead, all of whom died thanks to the humble Cape Snag, return to the story's current timeframe in ways that are both epic and well-earned, with Apogee and later Macroburst and Phylange following soon after. And as for the originalSuper Family Team, who are already a Badass Family as is, a Noodle Incident or two before the beginning of the story has apparently made them even more badass.
In Queen of All Oni, Jade has done this, having gained increased strength in her new Shadowkhan body (and gaining a knowledge of shadow magic)]], and having put her genre savviness to good use against her former loved ones.
Later in the fic she plans to take MORE levels by finding three magic tablets she can use to increase her magical knowledge further.
The Ultimate Evil: Valerie is already somewhat badass at the start of the story, being able to fight off all three Enforcers with a broom. However, she's still out of her league, and realizes it, so she eventually asks Jackie to teach her martial arts.
In Horseshoes and Hand Grenades, most of the minor characters in the show gets the chance to shine. For example, former Zodiarts Motoyama and Norio become Kamen Riders (Den-O: Sword and Den-O: Wing respectively), while Butt Monkey Chuta Ohsugi beocmes a Badass Teacher willing to take down Dustards, his former love interest and the Big Bad with nothing more than sheer guts and determination. Likewise, comic relief characters like Yuki can now punch a Dustards' light out with a "HAYABUSA PUNCH!"
In Trip Like I Do, Dave and Mindy run away from home and undergo near constant training at one of Mindy's safe houses. Dave becomes far more physically fit (being able to engage in Roofhopping like Mindy), fights much dirtier (due to being promised a day off if he can take her down), and is a much more skilled fighter overall.
The Immortal Game: Pretty much all of the Mane Six over the course of the story, but the crowner has to go to Fluttershy of all ponies. During the Final Battle, she somehow taps into Terra's old power as "Princess of the Forest", becoming a Physical Goddess with a flowing mane and something akin to the Royal Canterlot Voice who is able to cow the most powerful dragon alive into a Heel–Face Turn and serving as her mount in a Big Damn Heroes moment. Everyone else (both in and out of universe) is shocked at this, and even Rarity admits to being a little frightened of her.
In the Pony POV Series, its easier to list the characters who don't do this. By the end of the Princess Gaia Arc, the mane cast and Trixie are tough enough to fight a Mad God head on and hold their own. Derpy Hooves beats down the mooks of the Big Bad to protect her daughter and is an experienced Companion of the Doctor, Shining Armor is a trained military warrior who managed to fight out of a full on military ambush, Princess Cadence took out an Avatar ofHavoc in the back story (though she can't remember) and is willing to put her life on the line to protect Shining, and the CMC managed to build a functioning time machine. Heck, even Celestia and Luna are complete badasses in this continuity.
The G1 Mane Cast get this as well, the Rainbow of Light having made them The Ageless. What do they do with it? Spend several thousand years kicking the flank of any monster who dares threaten Pony Kind. This includes managing to beat Discord in a straight up fight (though unfortunately, he managed to escape) and beating back Grogar (who in this universe also counts as this trope, being a Evil Overlord who can go toe to toe with Discord) every 500 years. Of note are Mimic, who managed to fight Discord head on and ultimately break his tooth permanently in a Dying Moment of Awesome, and Shady, who was Discord'sThe Dragon and ultimately redeemed herself by holding him in place for the Princesses to beat him.
Some of the G2 Mane cast get this, considering they end up having to survive After the End when a Class 2 apocalypse hits. Sweetheart becomes a nurse who stops a supercharged version of The Black Death and her husband Teddy goes Papa Wolf and protects his family from corporate thugs and mutations spawned by the blast. Patch is the biggest, as she goes on a quest to rebuild the Rainbow of Light, becoming an Action Girl who overthrows a number of ponies abusing the shards' power. She even manages to beat a dragon in a straight up fight and outwit and beat Grogar. Did we mention she has no special powers outside the Shards slowing down her aging?
Minty Pie from Dark World. When we first meet her? A normal everyday Earth Pony (though one who grew up in Discord's Crapsack World). Then she decides to go help the others in the fight with Discord. She falls into an underground cave and being hailed as The Chosen One by a race of winter spirits. By the time we meet her again, she's defeated her own Big Bad and became an experienced warrior with a Cool Sword. By the time she reaches the Final Battle with Nightmare Paradox, she dodges some fireballs with practiced grace, throwing the time travel scroll that Paradox wanted to force Twilight into using into it in the process and manages to hold her own despite being the only Badass Normal in the group.
The Rise of Darth Vulcan: Vulcan's apprentice, Artful Dodger, starts out as a mere Genre Savvy street urchin crook. With training (and some magical body enhancements) from Vulcan and Chrysalis, however, and he's one of Vulcan's Co-Dragons, with a fearful reputation of his own amongst the minions.
In Blind, due to one failed recon mission (that was VERY important) Naruto, Gaara, Hinata and Shino take levels during the time skip. Naruto and Gaara had to due to being declared missing nin with kill on site orders in KUMO where they are hunted like dogs EVERYDAY so they had to toughen up or die. Hinata because of throwing herself in training due to heartbreak (Naruto didn't tell her about the mission and ends up as a missing nin so she takes it hard. Doubly so because they are betrothed) to the point where she can take Neji down in a fair fight. Shino because of anger at his friends being traitors (Naruto and Gaara) and one giving in to depression (Hinata).
Future Sakura in Naruto Mysterious Power, who manages to defeat over half of the main cast, went from being nothing but the main antagonist's sex slave to using the powers of eight tailed beasts and a cursed seal, along with sonic screams and various Chidori-Raikiri versions.
Naruto again in Legacy of the Rasengan: Naruto. Due to misunderstanding something Iruka said, (hearing "the 4th Hokage was a great shinobi WHO just so happened to create his own jutsus." as "The 4th was great BECAUSE he created his own jutsus.") Naruto decides to tell the Angst to fuck off and become a badass with his own collection of jutsu. The kicker being all his jutsu are simple and easy to use but WILL reach Gamebreaker status when he gets older. Examples of his jutsu:
Kaze Tate no Jutsu : Creates a tornado around the user that works to deflect things thrown at him. More chakra = Raging Tornado defense. Can also be used for offense (Naruto used it to stop a Gatsuuga).
Kaze Kiwa no Jutsu : Creates a wind sword that Naruto upgrades to be near Rasengan levels. Since it contains nothing but wind and chakra, its tip causes massive damage. Naruto used it to cut through Neji's Kaiten.
Nicely deconstructed in Escape from The Hokage's Hat by Naruto. Instead of just handing him high level jutsu or just working on his chakra control, Tsunade slowly turns him into a Badass Bookworm fixing his more Leeroy Jenkins impulses, forcing him to plan (he hasn't really got a choice due to a Game-Breaking Injury he's been recovering from for at least 20 chapters), working on his chakra control and getting him involved in politics. The end result is that Naruto goes from a Unskilled, but Strong bruiser who throws massive amounts of chakra, clones and Indy Ploys to solve his problems, to a Weak, but SkilledLightning Bruiser who can talk and plan his way out of a problem but also get into a fight with 3 Chunin and win by manipulating the fight to his advantage.
Played with in The Sealed Kunai where Naruto isn't so much taking levels in badass as regaining them by getting back into the kind of shape he was in as a ROOT member.
A Crown of Stars: Shinji and Asuka after getting much needed pshychological counseling and a brand-new Humongous Mecha. They go from two wretches barely capable to fight for their lives to commanders of an army.
Advice and Trust: All pilots take at least a level, but special mention goes to Rei who went from being the least effective (albeit most reliable) pilot to giving a good showing against Zeruel.
The Child of Love: Due to Gendo messing up with her unborn's daughter DNA, Asuka became fiercer and more dangerous when she fought. She single-handedly held back and pushed away a giant moster who almost crushed both Shinji and Rei.
Asuka goes through several phases of depression and recovery. Every time she overcomes her issues she becomes stronger and a better pilot.
According Kaworu, after recovering from her accident Rei became a better fighter pilot than she was before.
HERZ: After spending twelve years training, Shinji, Asuka and Rei are more badass than their teen selves. Shinji and Asuka train every day (Shinji shoots targets in the range, Asuka tries combat simulations inside her Humongous Mecha...) and Rei has an A T Field capable to destroy or pulverize anything -as several terrorists found out- and she has absorbed two ancient alien deities. Asuka is an army commander, too.
Higher Learning: Asuka spent a good while depressed and gloomy due to getting constantly beaten and even mind-raped. Shinji managed to make her snapping out of her depression and encourage her to fight right before the final battle, though, and she tore the MP-Evas apart.
Neon Genesis Evangelion: Genocide: After waking up from a coma, Asuka trains hard and becomes a best fighter, managing to kill an Angel which had beaten all remaining pilots.
Nobody Dies: A very interesting inversion. Shinji Ikari is nostranger toTaking Levels in Badass, but Nobody Dies Shinji could be more accurately described as having Lost A Level In Suck; thanks to Yui surviving the backstory, Shinji has grown up into a well-adjusted and perfectly normal teenager, with none of the numerous neuroses that plagued him in canon but also none of the unGodly self-confidence or super-powers other fanfics tend to give him when doing this. It makes for a very interesting take on Shinji's character to see him stick up for Asuka and trade verbal jabs with his classmates when he's not smiting Angels with an Eva-scale BFS or a Wave Motion Gun. Then again, when you're the walking embodiment of This Loser Is You, even becoming an Ordinary High-School Student is a big step in the right direction.
Shinji took a level when he returned to the past. Now he knows what the stakes are and what the prize of failure is, so he fights seriously and fiercely rather wasting time whining and feeling sorry for himself. And he uses all his experience and knowledge gained in the original timeline. The result is a more dangerous, more focused and more experienced fighter.
As shown in his first battle: in the original timeline Sachiel beat him around until his mother went berserker. In the altered timeline he slaughtered Sachiel so quickly and ruthlessly than Gendo and Ritsuko thought Yui had gone berserker.
The One I Love Is: Shinji tried to become more badass throughout the war to protect people he loved. As a result of it, in the Final Battlehe broke synch records and utterly destroyed the MP-Evas.
In The Second Try, Shinji and Asuka are more experienced and better fighters than their canon selves, since their immaturity and psychological issues aren't holding back their potential.
Shinji And Warhammer 40 Ktried to invert this trope in a manner similar to Nobody Dies but the author wound up overshooting the target and turned Shinji into a very straight (and very EPIC) example of this trope instead (Case in point, there's nothing ordinary about an Ordinary High-School Student with his own gang and the universal respect and adoration of his entire hometown).
Luffy in Nine Minutes loses his Devil Fruit powers at the end of the Alabasta arc due to temporarily dying. But by the end of the Skypiea arc, he's gained mastery over haki AND Enel's Rumble Rumble Fruit. It's bad enough that when a somewhat injured Smoker confronts Luffy while the latter is still injured enough to need a cane and Robin's support to stand, Smoker realizes he can't beat Luffy.
Likewise Usopp and Vivi both get Devil Fruits of their own. Usopp gets the Toad-Toad Fruit: Cane Toad Model while Vivi eats the Sand-Sand Fruit.
In Supernova, Luffy eats a Logia Fruit that turns him into a Sun Man and learns Rokushiki from his grandfather. He also goes on to teach is crew Rokushiki. As a result, Nami beats Krieg and Zoro manages to use Razor Wind by Arlong Park. By the time they reach Loguetown, all three have the highest bounties in the East Blue (50 million for Luffy, 30 for Zoro, and 20 for Nami).
In Chapter 19, when Tashigi asks Cross if there's any way to break Mr. 3's wax, he informs her that the only swordsmen capable of cutting anything are those capable of cutting nothing. A short while later, after Mr. 3 impales her, she manages to figure out what that means, cutting through Mr. 3 and his wax to defeat him.
Vivi and Carue, after joining the Straw Hats full-time, have new weapons made for them by Usopp, a pair of crescent chain-scythes named Lion Cutters for Vivi, armor, wing and talon-blades for Carue.
Luffy unlocks a prototype Gear Second (Gear 1.5) as far back as Skypiea.
And a prototype Gear Third (Gear 2.5) as far back as Navarone.
And Armament Haki at the climax of Enies Lobby.
Leo learns how to cut steel when Donny suggests that Mikey be the new leader of the TDWS.
Nami gets her hands on Ohm's Eisen Whip full of Iron Clouds.
All over during the Enies Lobby Arc:
Boss learns and develops his Full-Shell Style, his version of the Six Powers. Even the Six King Gun.
The Poké Wars series takes this trope and dials it Up to Eleven. Each and every surviving character becomes a badass fighter.
The Sun Soul takes this idea and turns it Up to Eleven... for Pokemon. No really, the author manages to take something as innocent as Pokemon and upgrade it into a grim, gruesome, and nightmare-inducing retelling, without going overboard on the crazy. Each character is leaps and bounds ahead of their original counterparts in terms of sheer badassery and so far the story has consisted mostly of adventure, mixed with politics and romance, with plenty of action and Anyone Can Die added in.
Ash Ketchum certainly qualifies. Trainer abilities and savant heritage aside, the lad is dangerous with a knife.
In the Portal 2 fanfic Blue Sky Wheatley goes from a Lovable Coward who can hardly even walk...to a Lovable Coward who is willing to sacrifice his life to successfully take down GLaDOS, taking his flaw of bad ideas and weaponising it.
Everyone did in Magica Madoka Veneficus Puella. All of the weapons can be spammed without hesitation. All the characters have enhanced strength and reinforced body structures. Oh, and Homura's a potential goddess.
Sailor Moon Millenials gave this treatment to Chibi-Usaof all people by having her come back from the future as a teenager and newly-christened Neo Sailor Moon. While not exactly a Future Badass, she was at least as useful as the other Senshi on a consistent basis.
In White Devil of the Moon the Sailor Senshi are becoming this due to getting trained by Nanoha. Most relevant? Sailor Mercury kills Zoicite on her own without support. In the most recent chapter, each of the senshi are able to take on superior odds against their opponents with minimal or no support and win.
Happens to literally superpowered character in Sailor Moon: Legends of Lightstorm:
The entire point of Luna taking Serena to Lightstorm in The Celestial Renegade was so that this could happen to her. The titular Lightstorm leveled her up with only a few hours of training.
Every Scout possesses new and more powerful abilities. Attack names have been done away with completely, Sailor Moon now uses her sonic scream as her primary weapon, Sailor Mercury now uses ice instead of water, and Sailor Venus fires powerful impact beams and can control six nigh-unbreakable energy chains.
Sailor Mars' powers are pretty much the same, but she can now use her powers to fire powerful jets of flame behind her, letting her "rocket-run" and rocket-punch" at insanely high speeds.
Sailor Jupiter made her debut in the original anime by beating up three thugs. In her first scene in Lightning Round, literally the first thing she does is kill six rapists, after a few hours of parkour, without even transforming. Her adrenalin-fueled rage was so strong that she began to transform without her Transformation Trinket, setting the events of the episode in motion.
In the episode's climax, Jupiter becomes the first and only "tagless" Sailor Scout (a Scout able to transform through sheer force of will) in history.
All the Scouts, in terms of fighting skills. Thanks to Lightstorm, the Scouts now fight off entire squadrons of Negaverse drones at once, and every Scout has had her fair share of kills (Even Mercury, who frequently impales enemies on gigantic ice spikes).
Tuxedo Mask has done this as well. His razor roses can slice through the toughest of Negaverse drone armor, he is powerful enough to knock one off its feet with one punch, and he is durable enough to cover Sailor Moon and protect her from an explosion. Inside a hijacked subway.
Even Jadeite and Zoisite have had this treatment. Zoisite now consists of two superpowered bodies, and Jadeite fights with all of the powers he had in the anime.
A Voice in the Wilderness gives several levels in badass to the Borg after years of Badass Decay in canon. The Borg are now shown using their capabilities far more intelligently, swarming their target, using transwarp to Flash Step around the battlefield, and even having a cube approach its target corner-first to bring weapons on three faces to bear. The upgrades are very nearly lethal to the protagonists. This is all enabled by the Borg Queen being co-opted by the Borg Collective's original AI, One of One.
The author allows Wondergirl a lot more time to actually do stuff in this fic: fighting the furies, rescuing herself from penguin's giant shark, fighting the hordes of apokalips, and even taking down Volcanna all by herself in the Final Battle. A big step up from her appearances in season 2.
Beast Boy goes from the goofball with some utility to the badass responsible for banishing Klarion from our dimension. The surest sign of this is when in the final battle, he becomes a dinosaur to fight Teekel on more even terms.
Superboy also gets a boost after he's partially aged to unlock TTK (Tactile Telekinesis like in the comics). It gives him a little extra punch to his strength, and even lets him fly.
Wally West gets a speed boost from being under Apokalips, allowing him to fight on even terms with The Flash when he was slower than him in the show.
Matthew Streika, once the living embodiment of the Idiot Ball carrying Wide-Eyed Idealist, now has taken some levels in badass. He threw himself in the way of The Brute's attack, got some nifty magic, and kicked a lot of ass. Shuuji followed in Chapter 2, redeeming himself by making a Heel–Face Turn, and then reducing Medrautto a quivering wreck. Even little Alina, who in the first Canon was an annoying Bratty Half-Pint, became so much more badass as a Biological Superpowered Magical Weapon. Alina's adoptive little sister, Alexandra "Lexie" Ivanov, starts out only able to summon little lizards and heal people...and she is a Shout-Out to Final Fantasy's Summoner Class...wait, what's she summoning now, a bigger lizard? Nope, you're dead, Adrian. What's that sound? Its the world taking a fear shit: She just summoned Dragon Emperor Andreclav