I don't think it'll happen, in that one main Mega Man is enough for most. But, I also think no one would outright be against it either. Most people would want a supporting character or two in after that. So it's usually any combination, 1 Mega Man, Roll, Zero and Tron. Who knows, maybe one day Bass or Protoman can also be in a fighting game.
Personally, I'd REALLY like to see it happen in some game one of these days. Each of the Mega Men could have vastly different fighting styles based on their weaponry. Volnutt's Stance switching, Mega Man's Keep-Away, MMX mixing in some Shoto stuff with his unique weapons as well. EXE could be based around his sword, though that's just scratching the surface of his tech. And the only two that have some semblance of being too samey is EXE and Starforce. Though even those two could end up being vastly different fighters with the right choice of weapons.
I mean, most fighting games have a character or two who has a "variant" of them anyway. Ryu/Ken, Nina/Anna, King/Armor King, Siegfried/Nightmare, Link/Toon Link ect ect.
That said, if ANY Mega Men were added in on top of Mega Man himself, I'd really want it to be either Mega Man X, or EXE.