So, Capcom has said that the response to the North American release of Tatsunoko Vs. Capcom could merit the production of a new Versus game. I personally would love to see another Marvel crossover, but in lieu of Marvel Vs. Capcom 3, I think an X-Men Vs. Street Fighter 2 would be a nice intermediary step. Sticking to only SF and X-Men properties obviously discludes a lot of favorites, but I think it would be wise to focus on a smaller roster while bringing tag battles up to date.
Marvel Vs. Capcom (1) had an interesting implementation to consolidate the roster. Ken and Akuma weren't included, but Ryu could spend a super bar to change his style (and palette) to something akin to his shoto-clones. I'd really like to see all three in a new crossover game, as differentiated as possible.
Another interesting quirk of X-Men Vs. Street Fighter (1) was that the characters from each side were generally paired up based on similarities. Ryu and Cyclops, Magneto and Dictator, Zangief and Juggernaut, etc. So, I says to myself, I says, "Self, if I were to pick a trilogism of X-Men to align against Ryu, Ken, and Akuma, who would I pick?
The Summers Brothers, natch. Cyclops is a match for Ryu's familiar stoic template, Havok shares the brash, reckless traits of Ken Masters, and Vulcan brings the same dark, mysterious elements to the table as Akuma. This would entail completely new designs for Havok and Vulcan, as they have never appeared in a Capcom fighter, which is a lot of work, but something I think would payout huge dividends with fans and fanboys alike.
That's a good start for a roster, but to expand on this...
These eight all have a history in the Crossover games, so they're a pretty solid way to flesh out the sides. And with the exception of Chun-li/Storm, it's in keeping with established pairings. Obviously the Marvel characters would need to be completely modeled, and animated, but their earlier incarnations should provide a framework for revision.
This next batch provides some fun pairings, and would include Rufus and C.Viper in a VS. series installment for the first time. Of the new SFIV characters, I find Ruf and Viper to be the best fit for the high-flying frenetic style of the crossovers. El Fuerte definitely has an exciting fighting stlye, but his reliance on wakeup mixups would not translate too well, I don't think. It would also mark Sagat's first outing against the Marvel universe, which would necessitate some retooling, especially if he is Sentinel's counterpart. Cable (along with Magneto, Storm, and Sentinel) is included due to his popularity and superiority in MvC2 and I've paired him with Charlie just for the hell of it.
And then here are some fun characters that aren't exactly necessary for a full game, but would always make great secret or DLC additions. Deadpool is included, of course, due to his immense popularity, and he's across from Ibuki, the best match for him in the SF world. And just as Deadpool rides in on Cable's coattails, Polaris does likewise slipping in with Havok, though I thought her and Juri shared some interesting elements. Jubilee and Sakura aren't characters I'd like to see per se, but they make for a natural pairing.
So yeah, a ton of glaring ommissions, I know. If you need Rogue in there for some reason, swap out Polaris. (It'd be less work that way anyways.) Sabretooth likewise didn't make it, though if you could part with Juggernaut or Sentinel, he makes for another big Marvel character. (Oddly, Sabretooth was paired with Dhalsim in XMvsSF, but for teleportory reasons, I feel Spiral is stronger.) If I felt the urge to include Omega Red, I'd probably make the case that Birdie (with his Murderer's Chain standing in for Red's Carbonadium Coils) should also be included.
And yes, again, this is just rampant fanboy speculation, and I haven't seen data yet on how fast Tatsunoko Vs. Capcom is moving. It may be a moot point, but I know I'm not the only one with a jonesing for a modern Marvel crossover.