Also, it helps most players remain in their comfort zone. If things get too drastically different, they'll get hesitant and be turned off.
Let's not forget that Capcom aren't the only ones who've recycled a game engine structure, recently. *cough*Nintendo'sSuperMarioGalaxy2*cough*
Nintendo's spent years making completely original Mario games in look, aspect and gameplay. If their very best Mario game of all time (widely regarded as, for you picky ones) requires an expansion, I say let them. As long as we're not still getting Mario Galaxies for years, I'm okay with it. But for one, although I love playing certain games and wish there could be more of them, I certainly hate playing expansions of the same game for years and years.
Also, time for more speculatan:
As Steve is finally given some gameplay importance in the next game, anyone thinks he'll play a little differently, or he'll be the same as regular Megaman gameplay, with just a skin? An ideal thing would be to have Steve-based levels, complete with ugly boxart enemies and bosses. At least a single level, just for fun.
...man, now I want a Steve game.