So....this is something I've been wondering about for a bit, but should Marvel vs Capcom 4 or whatever it is called ever gets made (that was rumored a while back as I recall) and things with the game itself goes well enough (so there is good will going ahead)...how would the general reaction be for DLC fighters that aren't quite Marvel or Capcom? 

...My main thought was that Marvel might use its ownership by Disney to borrow fighters for their DLC wave(s). An example being something like Darkwing Duck whom I don't think would be utterly out of place in the game. Could be anything really: Disney owns a lot of stuff. 

Ton of other fighting games get fun out of such things, and I'm curious if Marvel v Capcom could do something similar. At the very least it would probably get headlines. 

They should focus on bringing back classic characters moved out of the way due to Disney pushing, before thinking of any guest character.

Perhaps I'm simply operating under the assumption they'd try to emulate the idea of 'larger rosters' and thus cover that by default. 

They better get back to the drawing board for the next Marvel vs Capcom. That Infinite crap wasn't the business. Especially with the very noticeable absence of X-Men and Fantastic Four characters. It wasn't good in my opinion. I really hope we get a better and well thought out game this time.

Remake MvC2 and keep it 2D!

I think fans of that series would rather they stick to Marvel and Capcom characters.

Not going to happen.  Capcom already stated when developing the original Marvel Vs. Capcom 3 game that the game will focus on marvel and capcom, and not disney, or any other property disney owns.  The game is Marvel Vs. Capcom, not Disney Vs. Capcom.  Especially with how much negative press MvC:I got on top of being shown up by Dragon Ball Fighterz, there's no way they're going to risk another screw up by adding Disney characters into the mix.

Given what the series has gone through, and what Capcom has already said about disney characters, it's a 0% chance of happening.  Adding outsiders to a Marvel Vs. Capcom wouldn't be Marvel Vs. Capcom anymore.

Also very unlikely.  Marvel Vs. Capcom 2 is broken, and there's no way that a remake would be exactly identical to the original game.  It would be ignored, in favor of the Dreamcast & Arcade versions.  Much like what has happened with Street Fighter II & III.  The best thing Capcom could do, for something like that, is to just make an entirely new entry.

No sense going back to Marvel Vs. Capcom 2 when only 8 characters are viable in a tournament, and rebalancing the game would make it not MvC2 anymore.

The best they can do for MvC2 is to re-release the game, and have it actually be arcade perfect.  The PS3/360 version was close, but even still, it had some serious issues.

And not only that, but only giving us 5 new characters at launch while locking the rest behind a paywall was pretty shitty as well.

But to answer the OP's question, no. It's called Marvel vs. Capcom, last I checked. Guest characters work for other fighters like Mortal Kombat, but not MvC.