Current candidates for who I might submit today in SMSB:
MIPS (Hook would be having a moveset based around speedrunning, specifically SM64 speedrun glitches and exploits)
Mecha Bowser (Hook would be being a stance fighter where up, down and side specials let you control the head, legs, and arms respectively)
Big Yoshi (Hook is that it's Big Yoshi)
Toad (Hook is his very accurate SMB2 physics, you know how SSB4 and Ultimate make Mega Man feel exactly like he does in the games? Yeah like that, but since Blue Toad would be a speedy grappler it actually isn't annoying or anything)
Dry Bones (Hook is that he can survive much longer than any other character because sometimes you might only KO part of him, like the head and arms, but the legs and body are still there to fight)