Hmm... Here's who I see as potential DLC for each Series. Only rule, they've to not appeared as anything higher than an assist trophy. If they appeared as a normal trophy they can slip past.
Any missing Brawl Veterans
Mario Series: Captain Toad, Bowser Jr
Legend of Zelda: ???, Impa?
Pokemon: Mewtwo, Sceptile,
Donkey Kong: Dixie Kong, King K Rool
Kirby: Bandanna Dee
Star Fox: Krystal
Metroid: None (Ridley is either playable or a boss)
Yoshi: None (Debu my foot)
Earthbound: None
F-Zero: Black Shadow (if Ganon's original moveset is dropped)
Retro Nintendo: Takamaru,
Fire Emblem: Anna, Roy
Wario: None
Kid Icarus: Medusa, Hades
Pikmin: Any of the other captains
New Series: Wonder Red, Inkling (Splatoon)
My top 5 from that are Mewtwo, Bandanna Dee, Takamaru, Dixie and a newer character.