I've never been much of a fan of
Geno getting into Smash, but a mechanic that I thought would be incredibly cool for him would be "
timed hits," where pressing the A Button when one of your attacks comes into contact with another character would give the attack some boosted damage and knockback. Although, I admit that this idea is blatantly stolen from the absolutely fantastic Legacy XP mod for Brawl.
You can see how timed hits work with Geno right here (when the stars fly out and the sound effect plays), and they're incredibly fun to pull off. He's one of my favorite characters to play because of it.
Another thing I would love to see would be for any
Rhythm Heaven character, were one to be in Smash, to have somewhat of a rhythm meter. As in, preforming multiple attacks in a certain pattern or to a certain rhythm would result in attacks becoming stronger, or something of the sort. Although, I feel like it would be very difficult to implement, for each stage has a multitude of unique songs with varying speeds.
I've always thought that if Pichu came back as an extreme 'glass cannon' clone of Pikachu, I'd be totally down for him returning. Like, his attacks would come out incredibly quick and hit harder than Pikachu's would, but he would have extremely low death percents.