Alright, maybe not Mega Man specifically, but Super Smash Bros. designer Masahiro Sakurai says the inclusion of Capcom characters in the next installment of the series isn't out of the question. The mere utterance of the word "Capcom" from Sakurai's mouth is all we need for hopes of Mega Man's appearance to rise:
"I can't say that it's entirely out of the realm of possibility that some Capcom character could appear in the next Smash Bros. The big problem, though, comes from the idea of trying to get characters from a completely different universe to fit with the style that has been dictated by Nintendo's characters in a fighting game." - Masahiro Sakurai
Mega Man's style, from any series, definitely adheres closely to the Nintendo mythology in terms of design and character. I won't say classic Mega Man specifically, but it was easy to mistake him for a first party Nintendo character back in the day. Classic Mega Man is pretty darn synonymous with Nintendo, if you ask me.
Sure, other Capcom characters could work in Smash 4, but, really, could you see Leon S. Kennedy or Jill Valentine going at it with Mario and Kirby? Er, maybe... but Mega Man certainly seems like a better, more appropriate fit amidst the already famous SSB roster.
Source: IGN (via GoNintendo)