I wanted both Mega Man and Bass in the game. My idea for Bass was to be based off his Mega Man & Bass weapons, while Mega Man is based on Powered Up. But if Mega Man is based on Mega Man & Bass as well, then both characters can be each other's semi-clones.
The Mega Man & Bass attacks could be:
-Copy Vision (Astro Man's weapon)
-Wave Burner (Burner Man's weapon)
-Remote Mine (Pirate Man's weapon)
That leaves only the matter of what the 4th attack would be:
-Tengu Blade (Tengu Man's weapon) <-- I was going to add this, but Remote Mine essentially does the same thing.
-Spread Drill (Ground Man's weapon) <-- Once again too similar to Remote Mine.
-Lightning Bolt (Dynamo Man's weapon) <-- It's a screenwide attack, obviously doesn't fit except as a Final Smash.
-Ice Wall (Cold Man's weapon) <-- Its uses might be too fringe to warrant inclusion, but possibly worth a look.
-Magic Card (Magic Man's weapon) <-- Short range projectile.. Doesn't seem like it would do much.
So maybe for Mega Man, the 4th attack would simply be Rush Spring.
For Bass, the 4th attack could simply be one of the above weapons. (Or Treble spring.)
There is MM's slide and Bass's dash, but maybe those could just be their dash functions. They can still be attacks, this could be triggered simply by pressing the attack button while using them.
Bass's final smash would Treble adapter form which appears in MM8 and Mega Man & Bass. (as opposed to his more simplistic looking MM7 Treble adapter form) I think his attacks while in this form should be specifically based on his Mega Man 8 version.
(Copy Vision) MM hologram fires shots for you while you go off and do other things. And you can still fire while using it!
(Wave Burner) Flamethrower style weapon, rack up damage or keep opponent at a distance.