Now this is pretty awesome, but what games have say, League characters appeared in besides League? There still are many games that might be untouched. Regardless, Ryu truly is the Kevin Bacon of the gaming world it would seem.
Yes, you'd definitely run into several major franchises or rosters that are isolated and as yet untouched (I can't see how to get to Blizzard, for instance, as they don't really lend their characters elsewhere), but even with fighting game crossovers alone you can still get quite far.
For instance, Link gives you Soulcalibur, which gives you Kratos (GoW), who gets you to both PSASBR (which you can also get to via Dante) and Mortal Kombat (2011 on PS3), which delivers Injustice. There's your Marvel-to-DC link right there that also gives you a number for Batman (Ryu–Dante–Kratos–Scorpion–Batman), an upper bound of 4. And maybe I'm missing something obvious that is even more direct.
Hell, Soulcalibur's guest rosters get you to Star Wars, which yields all of Masters of Teräs Käsi. Ryu–Link–Ivy–Vader puts Luke Skywalker also on 4.
If we limit ourselves to fighting games, it's actually hard to think of any major title totally disconnected from Ryu. Primal Rage, maybe? Killer Instinct (unless there's a path there via Battletoads)?
[Edit: I've spun this off into
a thread of its own.]
