I see where you are coming from, but there are holes.
When you are doing a crossover, you should have a good chunk of heavy hitters, yes. However, if you are a franchise like Smash Bros. or Marvel vs. Capcom, at a point you want to add in some odd ball characters to spice things up a bit. Even Smash 64 had an oddball choice with Jigglypuff and Melee had Mr. Game & Watch which defined the term "odd". Also, slight thing here, but I don't think Gambit is your resident oddball pick. He's one of the more popular X-Men.
Marvel vs. Capcom's closest game to "the gang is all here" is Marvel vs. Capcom 2 with everyone from Children of the Atom, Marvel Super Heroes, X-Men vs. Street Fighter, Marvel Super Heroes vs Street Fighter and Marvel vs. Capcom: Clash of Super Heroes all in the game, giving it room to add in characters like Marrow or Anakaris or B.B Hood while Marvel vs. Capcom: Infinite had no idea what it wanted to do with it's roster (especially Capcom's side). Marvel vs. Capcom also has it's fair share of oddballs and flat out bizarre characters (have you seen Norimaro from MSHvsSF or Anita from Marvel Super Heroes?).
MvC:I was the mess it was because Disney really wanted to be seen as the head honcho in terms of demands and how the roster was going to be, at least in the Marvel side. But it was also a mess because the game was strangely, and stupidly, a BUDGET title. Which means they couldn't do a lot with the money they had, which is baffling because you have support from Marvel and Disney, with their near endless supply of money. MvC:I is just an enigma.
Marvel 3 was the most experimental one in terms of roster additions which in my opinion, gave it the best and most varied roster in the series so far. While removing Mega Man and Venom was dumb, a lot of the characters introduced in Marvel 3 are excellent additions to the roster.
Marvel vs. Capcom is a franchise that with the right amount of freedom and negotiations, can go ham with the roster. Heck, they could pull a "Everyone Is Here" if they want. But the question are they willing to go that far? Now, both companies have the money and resources to do it. But will they? That's the question. I think MvC4 or MvC5 (if you count Infinite as MvC4) would be the best time to pull it off. They want to get back everyone they lost from the fanbase due to MvC:I so what better way to do then to pull of a Smash Ultimate and make it the swan song of the series if need be.