When Worlds Collide (formerly Heroes United) is a three-way Crossover Fan Fic between the Super Mario Bros., Sonic the Hedgehog, and Mega Man (Classic) series written by Nintendoman01. It can be read here.
Culex, the Dark Knight of Vanda, and his master, the Dark Mage, have plans to conquer the Mushroom World, Mobius, and the Earth of 20XX all at once. However, as environmental necessities prevent them from invading the worlds on their own, they turn to Bowser, Dr. Eggman, and Dr. Wily, arranging a meeting and subsequent Villain Team-Up between the three with the intent on taking over after they do their work. But what do Mario, Sonic, Mega Man, and their respective allies have to say about that?
When Worlds Collide provides examples of the following tropes (Beware of unmarked spoilers):
- Alternate Universe Fic: For starters, the fic ignores the Archie Sonic Cosmic Retcon post-issue 251, and states that Sonic and Eggman lost their memories of the events of Sonic the Hedgehog/Mega Man: Worlds Collide as well.
- Big-Bad Ensemble: Bowser, Eggman, and Wily along with their various fellow baddies.
- Canon Welding: The fic mixes and matches different characters and concepts of the various Mario, Sonic, and Mega Man comics, cartoons, and games into one continuity, such as adding the Super Troopas from The Super Mario Bros Super Show!.
- Crossover: Between Super Mario Bros., Sonic the Hedgehog, and Mega Man (Classic). With hints of Banjo-Kazooie and Final Fantasy.
- Dead Fic: Hasn't been updated since April 24th 2018.
- Evil Knockoff: Metal Sonic and Copy Robot reprise their role from the original comic while Bowser Jr. takes up the role of Shadow Mario once again.
- Let's You and Him Fight: The villains' initial plan: trick Mario, Sonic, and Mega Man into fighting and hopefully destroying each other.
- Man Behind the Man: Culex and the Dark Mage manipulate events from behind the scenes.
- Pocket Dimension: Bowser, Eggman, and Wily use one as their main base of operations, dubbing it Bowser's Skull Egg Zone or the BSE Zone for short. This dimension links Mobius, Earth 20XX, and the Mushroom World together.
- Recycled Script: The fic hits most of the notes of the original (The villains make their base in a pocket dimension, they send dopplegangers to instigate a fight between the heroes, the heroes realize what's up and work together, the heroes' friends are kidnapped and roboticized, etc). Particularly, Yoshi's kidnapping and subsequent roboticization is almost note-for-note what happened to Tails in the original comic, and the scene where Eggman reveals Burnbot to his allies is lifted almost verbatim from the original cartoon. Justified (kinda) since the characters involved in the original crossover do not remember it outside of vague memories.
- Antoine is revealed to be awake, despite him being in a coma prior to the original storyline. Justified as Lost Hedgehog Tales chapter one (which hints that the Freedom Fighters would have needed the power of the chaos emeralds to heal him) was not out yet.
- Snively is shown to be working for Dr. Eggman again when he was show in the ending to the Scrambled story arc to be rotting in a Prison Egg for his treachery. Either it's the robot Snively Eggman gave the Iron Queen, or Eggman somehow changed his mind.
- Shard the Metal Sonic is alive and even pulls off a Big Damn Heroes moment. But Forged In Fire ends with Metal Sonic not only defeating Shard, but also destroying his Power Gem, which is Shard's power source. Additionally, the comic makes it clear that both Shard's and Metal Sonic's Power Gems (both of which were destroyed) were the last ones. It is handwaved as Metal Sonic proposes that the Freedom Fighters managed to find him in time.
- A minor one, but the usage of the Super Mario Bros 3 cartoon as well as the fic describing Bowser Jr. as "a Koopaling" seems to suggest that the Koopalings are Bowser's children in this universe, an idea that was rendered non-canon in 2012.
- Suspiciously Similar Substitute: Yoshi Man fulfills the same role Tails Man did in the original comic.
- Teeth-Clenched Teamwork:
- Downplayed with the doppelgangers as while Shadow Mario and Metal Sonic are not impressed with Copybot's arrogant attitude, they still manage to work together pretty well.
- Bowser is not happy about having to convince King K. Rool to join him.
- Unwilling Roboticization: But of course. Yoshi and Donkey Kong are turned into Roboticized Masters.
- Villain Team-Up:
- Bowser, Eggman, and Wily all join forces to take on their respective enemies and conquer their respective worlds.
- Metal Sonic, Shadow Mario (aka Bowser Jr.) and Copy Robot work with each other to instigate the fight between Mario, Sonic and Mega Man.