A Freezing Block of Ice

[StarSock64]

Oh my

Mega Man is known to use the Rush Jet and Rush Coil which could be an Up B (possibly something akin to Snake's up b or Sonic's up b)
Mega Man flashes colors while charging if you care about that
edit: ope I'm blind nvm
Mega Man slides across the floor

everything else he does is pretty much game specific. He has a ton of special moves, you could incorporate that in a "press b+a direction to change weapons!" or you could just pick specific ones. One of his stranger ones is the ball thing he has in 8 and mvc but I can't see that working well in SSB. One he uses in MVC is the tornado hold, basically a giant pillar of wind. ...That doesn't sound very SSB though does it? A frequent weapon is his shield weapons, particularly his leaf shield from MM2/mvc(again). The first time you press the button, it creates a shield. The second time you press it, it becomes a projectile and shoots forward. He has everything from elemental attacks to explosions so it's kind of hard to choose. And for some reason I can't think of the particularly interesting ones! He's used a flame sword before but I'm not so sure about that one. He has bombs that stick to walls. He has random acrobatic moves. Ah... He's used his friends Auto Eddy and Beat to help him, you could use them in attacks (auto uses a rocket lancher, eddy spits items or bombs out of his head, beat's a bird that can charge forward. Dunno what you could use 'em for but yeah!)

You could incorporate the rush adaptor, where he combines with rush, possibly for a final smash. Other FS options are the screen-filling attacks he has or the time stopping ones (or technically you could have him use the rush adaptor and then use those)

tl;dr rush jet/coil, sliding, a shield weapon, rush adaptor

EDIT: In MVC mega man uses regular punches/kicks so you could use those for attacks