I said I was going to show that most things that had crossovers at all could all be connected to each other, so here we go.
First, a recap:
If you read the list of multiverses, notice 2 and 3; Marvel and Transformers. Not only is there an obvious overlap here, but both multiverses even have a system of classifying universes, with the Marvel Transformers comics specifically having a designation in both. (Actually 2 in each, because of differences between the US and UK comics.)
That’s the most explicit case, but not the only one where the multiverses overlap. There have been Marvel vs. DC crossovers. I already mentioned Tekken characters appearing in a Digimon game.
It is implied that Kaworu Nagisa from Evangelion is a multiversal singularity or time diver; that is, all incarnations of him, including the ones from Super Robot Wars, are the same individual. The same thing may be true of Minerva X, from Mazinger Z, which would potentially make just about everything Go Nagai has ever written part of that multiverse as well. The connection between this and the rest is odd; Transformers has never been in Super Robot Wars, but there is a Transformers X Evangelion crossover. (Which could be what gave someone the stupid idea that Michael Bay would be a good director for the live-action Evangelion movie.)
And there was Planet X, a crossover between Star Trek: The Next Generation and X-Men. According to Star Trek’s canon policy, that’s not actually canon, but it’s official, so I’m mentioning it.
So just to make it clear, the geekiest six-degree crossover chain ever might be Mazinger Z -> Super Robot Wars -> Evangelion -> Transformers -> Marvel Universe -> Star Trek
And Star Trek is part of the Tommy Westphall universe.
But we don’t need non-canon material to make the connection, because of Teenage Mutant Ninja Turtles: The Next Mutation. (See, it is good for something after all.) Cartoon All-Stars to the Rescue exists, and in between Papa Smurf talking about marijuana, it has all kinds of characters meeting that wouldn’t anywhere, including some of Disney’s, and well, this exists. On second though, maybe that’s not any better than the Star Trek novel.
But since this is all just for fun anyway, let’s see where we can go if we don’t worry too much about whether official crossovers are really canon or not.
Scooby Doo has met Batman (more than once), which brings in most of Hanna-Barbera’s characters via Laff-A-Lympics.
Homestuck was in Namco High.
Pretty much all of Disney’s characters have interacted to some degree, and with Final Fantasy characters in Kingdom Hearts. (Also, Disney and Marvel characters appear together in Disney Infinity.)
Did I say Final Fantasy? Okay, also add Ergheiz, Tobal 2, Secret of Mana, Secret of Evermore,and (thanks to the Fortune Street series) Dragon Quest and Super Mario Bros.
I still hesitate to count Super Smash Bros. since it not only takes place in a world where people turn into trophies, but also was clearly not on the official Zelda timeline. But really, what’s the most important thing in Smash that’s not already connected anyway? Earthbound? Link was in Soul Calibur, Mario was the referee in Punch Out, Ridley appears in Dead or Alive, and Mario and Sonic at the Olympics seems more like regular crossover stuff. Even Pokemon can be connected without Smash, considering Pokemon Conquest and Warriors Orochi. And Fire Emblem is connected because Dante was in a Shin Megami Tensei game. (Assuming Shin Megami Tensei X Fire Emblem ever actually comes out; they announced that game way too far ahead of time.)
Yeah, Link was in Soul Calibur. So were Spawn, Darth Vader, and Kratos.
Did I mention Hatsune Miku was in Super Robot Wars UX?
What about anime and manga that don’t have mecha? Tatsunoko vs. Capcom brings in several.
Jump Ultimate Stars is a crossover between just about every major Shounen Jump manga, but I haven’t found a connection between that and the rest. Other anime crossover games are either similarly disconnected or don’t add very much.
In the possibly one-way canon department, we’ve got multiple connections now to the Wold-Newton universe.
If you take both the Tommy Westphall hypothesis and all of these crossovers seriously, that kid is the most creative person ever.