For theories about other subjects, see The Legend Of Korra Wild Mass Guessing Index.
The Blue Spirit will returnSomeone new adopts the persona of the Blue Spirit, possibly to play both sides of the Bending Revolution, or to act as a neutral party in the revolution. Perhaps even multiple people using the Blue Spirit, all working together in Republic City.
Related the above Blue Spirit theoryThe Blue Spirit will actually be a Captain Ersatz version of The Shadow.
We'll finally find out what happened to Zuko's Mom.If only in passing.
Sozin's Comet will return 30 years earlier than expected.The comet came a little too close during the last approach and was pulled out of its orbit by the planet's gravity. The new trajectory will bring the comet back into alignment in 70 years instead of 100 years and nobody will see it coming. With no prepared plans to exploit the comet's bonus to firebending absolute chaos will ensue as firebenders compete for dominance.
Tom Tom will make an appearanceYou remember Tom Tom right? No? You know, Mai's little brother?
Tom Tom and Hope will be married and have a grand kid Korra's ageBecause pairing two baby characters from two different nations years later and tying them into the story again is exactly the sort of thing Mike and Bryan would do.
Sokka's Sword will be a legendary artifact.Made from a rare material? Check. Forged by the greatest swordsmith in the world? Check. Belonged to one of the heroes that saved the world? Check. There will be legends about that weapon, it may even become a status symbol at some warrior's order.
The tsungi horn is the Avatar version of a Trumpet.We'll get awesome jazz music and a continuity nod all in one!
There will be a dragon in the series.Specifically a new born one, from that egg from 'The Fire-Bending Masters'. Because face it, that'd be awesome.
Someone will say "Are you two so busy fighting that you cannot see your own ship has set sail?"Iroh did say it should be a proverb
Airbending will finally have its own spin-offBecause earth (metalbending), water (bloodbending), and fire (lightning-bending) had their own, but not air.
Metal cages used to contain Earthbenders in the old series will be a museum relic.Unless the practice of metalbending is strictly secured by only police units, it would be safe to say that the cages occasionally used to snag earthbenders and hold them are obsolete. Even then, it's likely the latter would be true and be used as a callback since there should be some museum in Republic City.
"Yuan" notes have pictures of the Gaang on them.Similar to how real-world nations put Presidents (and historically significant people) on money. Additionally, they're colored orange, red, blue and green for the respective person, for it's different denominations.
Zuko's entry onto the scene will involve Lightning redirection.He hears that the City is in a bad way because of the Equalist uprising being in full swing. He enters the scene, an equalist with zapper gloves accosts him, and he casually grabs their electrified hand, redirects, and then zaps them away without much exertion at all.
Tahno will learn to fight with a boomerang.Now that he can't bend, he'll fight alongside Korra to beat Amon and try to get his bending back. Because really, how cool would that be?
Bloodbending will return to be used against the Equalists.Either Korra or Tarrlok will learn bloodbending, use it to defeat Amon, then wreak havoc on the rest of the Equalist movement. From there the Equalists have even more reason to lash out against benders for the violation while benders have an excuse to go on a witch hunt over the de-bending.
The hippie nomads are still around.They would have discovered Guru Pathik at some point and started eating onions and banana juice with him, thus lengthening their lifespans to coincide with the next avatar's.
Mako will show he is able to bend with blue flames.Either by harm done to Bolin or other means, he will enter a state of Tranquil Fury and fight with a cold demeanor to his opponent (either an Equalist or Season 2) and show he is able to conjure up blue flames if angered. Lightningbending is a common thing, so why not blue firebending among a few firebenders too?
Zuko will teach Korra swords.He'll show up and be very unhappy with Korra, because she thinks she's nothing without her bending. He'll remind that she's the Avatar, and that bending the elements is only a part of what that means. Instructing her in swordsmenship would give a skill Amon can't take away, as well as help get her get the mindset for Air training. For bonus points, Zuko gives her Sokka's meteor sword.
The giant statue of Aang functions in the same way as the Avatar statues from the first series ...Meaning its eyes will shine like a lighthouse when Korra first enters the Avatar State. That'll provide the clue Tenzin needs to hightail it over to the city and calm her down before she causes too much collateral damage. Or, Korra will have to return to the statue, the site of her greatest defeat so far, in order to commune with Avatar Aang as he first did with Roku. Or both.
Aang will make an awesome Brick JokeKorra will go to Aang in search of answers for Avatar stuff. She'll ask questions that he can't answer, because it's the kind of thing that she has to answer on her own. Aang will remember how much he hated getting those answers from Roku, so he'll try to distract her... with the same marbles from episode three. After this fails, he'll give her the dissapointing answer.
Korra will learn to track and find people by spiritually locating them.This one was so rarely used in the first series, it seems to be forgotten. Both Aang and Guru Pattik used this method to find someone by using someone that they were deeply connected to. Tenzin will likely show Korra this trick by spiritually aiding her find a member of the Krew that was kidnapped.
Tenzin's unborn airbender child will be a son named Gyatso
Korra will first converse with Aang on the Vernal Equinox.Aang first conversed with Roku on the Winter Solstice, the beginning (or midpoint, depending on reckoning) of the season opposite Roku's native element. Korra, in turn, will do the same on the Vernal Equinox.
Azula will make her return as one of Korra's cell mates.Korra's going somewhere where "no one will ever find" her, likely a high security prison. Whether Azula's gotten saner or not won't matter if she decides she belongs there.
Tarrlok got the Equalist items from the evidence at the Metalbending station to do his cover-up.In the episode 9 preview, Tarrlok is doctoring the scene with a few items the Equalists use. Obviously, he is using the items the police and task force confiscated from arrests, bonus points if the glove was Asami's.
Amon is Koh.It's why blood bending doesn't work on him. He is pursuing an ulterior motive with connection to the spirit world. This is his mortal form.
In Amon's mass debending, we will see closeups of several familiar benders.Skoochy, Toza, Saikhan, and various other benders we seen in cameos or other scenes.
Yakone and Tarrlok's blood bending can't be performed on full moon because they're not actually drawing upon Yue to do it.There are spirit world shenanigans afoot, and one of the spirits is giving them more power than they should naturally possess. Yue is not cool with that, so as her power grows Yakone's/Tarrlok's weakens.
Azula is the firebender responsible for various orphans and facial injuries throughout Republic City.She's getting on in years, but still has her fun.
Azula founded and maybe still leads the Agni Kai triad.Related to the above: She might not have done all this damage personally.
Zuko will have adopted some of Iroh's traitsSuch as his love for Pai Cho, tea, and leads the Order of the White Lotus.
The Ember Island Players will make another appearance with another terrible play.Their leader will be a cartoon version of M. Night Shyamalan and the episode will be filled with Take Thats against the widely panned live action film.
There will be a movie about Korra's adventures.Instead of another play, there will be a movie, which incidentally will also be the first actual movie ever. Amon will have an evil mustache.
Sokka's Submarines will show up.But with waterbending operation an optional thing rather than required. Good chance the Equalists have them.
The Jasmine Dragon will returnEither as a humble tourist attraction with great history where it originally was in Ba Sing Se, or as the brand-name for the largest, high-quality tea branch in all the world!
Yakone's wife is Katara's second cousin.They do look alike, and Katara's grandmother is from the North Pole. Perhaps Tarlok's wife is descended from one of her siblings.
Hama didn't invent bloodbending.Rather, she was simply the first Southern waterbender to learn the technique, as well as the first female— males of the Northern tribe had already been practising it for centuries, it's just that a combination of their isolation from each other and the forbidden nature of the technique kept knowledge of it from spreading between tribes. Yakone mentions that he comes from a long line of bloodbenders, and that would be impossible if it never existed before Hama first used it. In her case, she just happened to have the same idea, and since she lived in the Fire Nation for her whole life after escaping from prison, she never learned of the existence of other bloodbenders.
The story Jinora read was actually a play, namely...Love Amongst the Dragons, the one Zuko said was butchered by the Ember Island Players!
Guru Pathik is still alive.Probably very unlikely, but it is possible that he's still meditating somewhere and might appear this series.
At some point, Korra and the team will stumble across Wan Shi Tong's LibraryMaybe Sato Industries, now under Asami's control, will be doing some excavating/building project in the area or maybe Korra is spritually drawn to it for some reason. But Korra ends up healing the rift that Aang and the gang caused. Because what would be more perfect to show the character development she's gone through (and will likely continue to go through) than to have her repair the spiritual relationship, between humans and a spirit that thought humans unworthy to share his knowledge, that her more spiritual predecessor fractured (especially since Korra embraces the side of humans that Wan Shi Tong holds little regard for whilst Aang SHOULD have been the ideal sort of human who showcased Wan Shi Tong's ideals).
Korra will meet Koh.Koh tells Aang, "We'll meet again." He seems to know things magically because he is a spirit, such as knowing that Aang had already met Twi and La. Also, during his conversation with Aang he indicates that he thinks of all the Avatar's lives as one person when he greets Aang with, "The Avatar. It's been a long time." As far as we know, Aang never saw Koh again in his life, so maybe Korra will run into him.
Tahno will join the Fire Ferrets in Book Two.The released preview animatics for Book Two reveal Bolin and the two new benders for the Fire Ferrets (Hasook doesn't seem to be there) getting their asses whooped. If training doesn't work out, maybe it's time for another Waterbender lineup change. Korra is the only person known to undo Amon's "permanent" chi blocking. In exchange for getting his bending bending back, Tahno has to work with Bolin, resulting in tension given their history last season.
Book 3 will be named "Ancestors".And we will find out what happened to the original Gaang through a series of flashbacks as Korra goes on a journey to reconnect with the past avatars, since she lost the connection at the end of Book 2.
Iroh ended up in the Fog of Lost Souls while searching for his son.He seemed to know exactly where Tenzin needed to look for Jinora. Further, Iroh's moment of self-realization, apparently a necessary part of escaping the fog, was probably what really triggered his Heel-Face Turn.
During their travels through the Earth Kingdom, the Krew will pass by Chin Village, possibly during Avatar DayThe only disappointment will be those Avatar dolls of unfried dough.
Lin and Toph's falling out was caused by Values Dissonance.To Lin, sending Suyin to live with Toph's parents was the action of a Crooked Cop, but Toph considers living with her parents as bad as jail.
Ming-hua is a Mental BloodbenderBecause her parents found out about her conventional bloodbending potential and chopped off her arms so she's now spending the concentration meant for Bloodbending to Waterbend with that level of dexterity.
There will be an "Ember Island Players"-style episode in Book 4Except instead of a play, it will be a Varrick production working to recap the last 3 seasons and it will be hilarious. It will downplay the highlights of the series thus far and, as a final dig towards the messy romantic plotline of the first 2 seasons, over-exaggerate them to epic proportions.