64: Going from the Smash 2 poll, the most requested characters were Bowser, Peach, Wario, King Dedede, and Ganon(dorf). If DLC was a thing since the N64, I feel that Bowser, Dedede, and Mewtwo (who was just behind Ganon(dorf)) would’ve made it in without the ballot since they were worked on during development. If a ballot did give us a new character, I’d probably say Peach since she was second, and I could see Sakurai wanting to add another female character.
Melee: 64DLC complicates things since Dedede never appeared in Melee. I could just pretend that Melee gets a delay to fit in the 26 canon characters and Dedede, but I could see someone getting dropped (I’ll say Game & Watch since I‘d like to keep two unique gimmicks). In terms of DLC now, I’d say that Wario, Toad, Diddy, Sonic, a few pokemon characters (I’ll go with the trainer, Charizard, and maybe a unique pokemon from Jhoto), and maybe Banjo-Kazooie would be the big requests. I’m confident that Wario wouldn’t need the ballot since he was the next unique newcomer to add. Also, I’ll say Game & Watch gets added because of his quirky nature. The final DLC characters I‘d say are Snake due to Kojima and then Sonic for both the ballot and the request from Sega during development.
Brawl: For convenience, I’ll keep Brawl’s roster the same even if some development time is freed up from Wario, Dedede, Sonic, and Snake already being in this hypothetical. Popular requests during Brawl to For were Mewtwo, Ridley, K. Rool, Krystal, Bowser Jr., Mega Man, and I think Little Mac (I’m sure Palutena and Shulk would be requested if it was for For and not Brawl DLC). For DLC, I assume that Mewtwo, Roy, Dr. Mario, and maybe Toon Zelda/Toon Sheik would be added without the help of the ballot (Dixie was supposed to be Diddy’s partner, and I’m confident praimai is just the random button). From a ballot, I’d probably say Mega Man is the one who gets added or one of Krystal/Bowser Jr./Little Mac if copyright issues are a thing.