I'm gonna have to agree with the people who suggest Eggman as a character. I mean, if you think about it, isn't he the second most reoccurring character of the whole series? Aside from Sonic and the Black Knight, I can't physically think of a Sonic game that Eggman does not appear in.
This Brawl hack shows that he is not too big for the game in any sense, especially at normal height. Then of course, they could go the route of having him fight in a very compact mech, probably no taller than himself, a la
Sonic The Fighters:
In fact, as people in the thread have discussed, I wouldn't actually mind Eggman being in an upgraded version of this very mech, but in a Modern Sonic style, with design features borrowed/upgraded from both the Egg Walker (
see here) and the Egg-Robo (
see here), topped off with a Metal Sonic-style propulsion system installed in the back of it. Most of his physical moves would be ripped straight from Fighters, and his specials would include, among other things: Egg-Robo's Beam Gun, a
very good up B, and maybe the ability to spawn Badniks (Dedede style).
As far as little visual details go, he should be wearing his new attire but his mech should have the classic Eggman look adorned onto it, with the upside-down yellow triangles on a red base colour. He'd have to have an Eggman Nega alt colour scheme, obviously.
Aside from having a wealth of moves borrowed from other mechs though, Eggman's main redeeming feature should be his incredible aerial game for such a slow and heavy character. He should have multiple jumps using his mech's jet, a glide ability (to 'hover' across the stage, in a very classic sense), and a very promising recovery move.
The Smash team love their little details, so you know they'd pour a lot of love and care into a new, custom Eggman mech that represents the series as a whole.