Good to see another die-hard Mega Man fan, heheh. Then again, shouldn't really be expecting much else in that thread, huh?
Personally I'd like to have at least one Mega Man 9 power, since MvC2's Mega Man moveset had two Mega Man 8 powers (which was of course the most recent game back then) but none of the MM9 powers seem all that feasable.
I think Tornado Hold would be an excellent move. Rock Ball was pretty cool both in MM8 and MvC2 but most Smash stages don't have walls and it would be pretty redundant with the Soccer Ball item as well. Leaf Shield is also pretty popular and rounds him out pretty nicely.
I guess my runner-ups would be Top Spin because it could make for a pretty neat "back off" move. Kind of a 5% damage, set knockback semispike move that would push the enemy away like 1/4 of Final Destination's distance? It'd be kinda like Fox's reflector in mid-air but without stopping your momentum and more horizontal than vertical. Sword Man's Flame Sword would also be pretty cool for a powerful close-range attack. Snake Man's snakes would be pretty lulz. But who am I kidding, I'm just mentioning them because, well, I always found it to be such an absurd power, yet he makes it look cool.
I'm a bit torn between Rush Jet and Flight Adapter. Rush Jet is old school but Flight Adapter is so god****ed cool, and if someone ever were to make Bass it would serve as a counterpart to Bass's Treble Boost.
Basically I'd like to see him end up with 3 non-redundant power-ups. One direct projectile is good enough, especially when it's his signature attack. The other 3 powers should serve different purposes.