Tekken vs Street Fighter imo would be virtually impossible as a purist fighting game, and "if" it was, I believe it would be built on completely different confines.
You simply could not make the game without drastically changing things for either the Tekken characters, or the Street Fighter characters. If you make it 2-D, how would characters like Jin etc, be able to utilize their SSing in those games, with things like WD that would be possible, however the Tekken characters would be at a significant disadvantage here.
Because they don't have very good mid-range effective pokes, just think, their pokes now, Ryu would mop the floor with them, because he's got great far reaching pokes like c.mk, c.fk, etc, etc. Which would make it extremely difficult for Tekken characters to try and get in on, then you add the fact that SF characters have projectiles and characters like Jin would have problems right there.
However, if you put them in a 3-D plane, then the Street Fighter characters would have problems now, because Ryu's footsie game would no longer be as effective as it once was within a 2-D plane, not to mention his Hadouken, which could literally be side stepped with ease, "unless" you made projectiles fly at a far, far, far, far greater speed.
It's been known that Projectiles in a 3-D realm are quite pathetic, not to mention, nearly all of Ryu's Specials would be useless, how effective would Shoryuken be, in a 3-D platform like Tekken? It would be virtually pointless as an anti-air, because almost nobody jumps, and how great would hurricane kick be?
Guile's Sonic Boom would need to be amped up in speed with tremendous results, otherwise the slow speed of it being an effective strategy is useless on a 3-D plane, because of SS, and the accessibility to get around it. The only thing that could "work" would be the button lay outs, because although Tekken would only have 4-buttons, while SF has 6-buttons. You could simply make the 6-button lay out, however playing as the Tekken characters, they can just use four out of the six buttons on there.
However overall there are just so many problems with a Tekken vs Street Fighter, that I don't see how they could truly "work" it without devaluing aspects from either the Tekken engine, or the Street Fighter. The reason being is, that in Tekken, you are forced to playing close quarters because that's what you can do, while in Street Fighter, you have long, mid, close range strategies, and by leaning towards either side to much, you'll force problems and devalue things from either side too significantly.
This would result in you simply either angering the Tekken players, or the Street Fighter players, if you choose to "work" with one engine more so then the other.
I honestly don't see it happening unless they just do something that's "fun" like a Power Stone type of game. Otherwise as a "serious" fighter, I highly doubt it being able to work, because the problems that would arise from what I stated above.
It would be 100x more plausible to make a Virtua Fighter vs Tekken, then the a SF vs Tekken.
