Marvel vs. Capcom is a contender for one of my favorite series, I think. There's a lot of optimism in the community right now and for whatever reason I'm feeling really confident something's going to happen soon.
I feel like some of those "banned" characters would almost have to make it. There is a lot of legacy to these games and their characters and I feel like a brand as big as Capcom could pull off more negotiation than they're given credit for (they got characters like Shuma and Sentinel in 3 based on their appearances in past games alone). I don't think anyone was grabbing their torches and pitchforks over no X-Men or F4 in Disney Infinity or Lego Avengers or whatever but there would definitely be a ton of backlash without Wolverine, Doom and Magneto making it into a 4th Marvel vs. Capcom game. I'm sure Capcom would do their best. That being said they were content ignoring requests for Mega Man, Gambit, Venom and a handful of other really popular veteran choices and 3 still turned out to be an incredible game so I'd want to see them try to do the game anyway.
The only really big change I'd be adamant about seeing is some kind of change to the assist system. The reason so many teams in 3 (and 2 as well I suppose) look the same is because of that "assist" slot on team usually populated by Doom or a handful of others. With a total of 150 assists to choose between in the game asking for them all to be great is laughable, but every character should have at least one useful move imo. Maybe the solution is to give every character only one assist ala Tatsunoko, but even then there would inherently be a few that make a lot more sense to use than the others so I don't know if that would solve the character variety problem 3 faces these days. I just know it's a really complicated system to balance but that a reworking of it would freshen the game up tremendously.
Other than that I just want to see something similar to 3. The same comic-book artstyle and awesome animations/effects (albeit with a touch more detail, maybe) would be just fine. A couple touches from other fighters on the market to round it out a bit more (I hope SFV's 3-4 hour story mode becomes Capcom's standard in the future) and some strong in-game tutorials would make it feel like a really complete product.
I don't really know about what characters should be in it other than that I'd die if we lost some of Capcom's newcomers from 3. I'm sure Phoenix Wright, Dante, Frank West and a lot of the others would come back but I'm kind of scared about characters like Viewtiful Joe and Amaterasu who are fading from relevance. Asura is probably a shoe-in and classic Mega Man is getting more and more popular so I'd be surprised to see him left out again. What's going on with RE these days? Chris, Jill and especially Wesker were all awesome but I have no idea who's relevant and who's not in the newer games.
I'd miss Wesker a lot :[