I love seeing people's guesses/hopes. Gets me even more hyped up for when the rest of the DLC characters get announced (I'm always happy to get more, even if it's an off the wall character or someone I didn't think of or care about). With that said, here's my (safe-ish and grounded in some sort of realistic guessing) list! It's made with the idea that 3rd party companies only can only take one spot for DLC (like you can't have Tails or another Sonic character since Joker is from Sega/Atlus).
1. Geno from Super Mario RPG - I've just always held out hope for some Super Mario RPG representation since Melee, and I think he'd be great. Plus, I want an amiibo of him terribly bad. I believe I've read SE owns him, and with how everyone has said what a pain they were to get on board with Ultimate, it's probably not very likely.
2. Phoenix Wright from Ace Attorney series - It'd be great as a cross promotion for the Ace Attorney trilogy coming out in the spring on Switch. He's already been in one fighting game, and I feel like this one would fit him even better. To be honest, I'd rather have Apollo Justice instead (he could have slow down mechanics like Bayonetta), but I'd be happy with Nick.
3. The Prince from Katamari Damacy - I honestly don't know how he'd work besides like rolling over people, but I love the little guy and the re-release of the first game brings him at least a little bit into people's minds again. Heck, I heard a segment on NPR today talking about Katamari Damacy and how great it is. I guess the only reason this may not work is because Pac-Man has The Prince as one of his items, I think?
4. Spot held for some announcement of a Nintendo title of some sort - I figure maybe they'd use their last spot to showcase someone from an upcoming game (new FE, Pokemon, Metroid, whatever character). Like they could do some sort of Direct and then it does a reveal like the inklings got for Smash. I dunno, this one isn't very creative, but it's safe.
I really wanted to come up with another spot for Konami, but all I could think of is wanting someone from Suikoden (Card Stories and Tierkreis were on Nintendo systems, though it's not a requirement since Joker is coming) or being lame and wanting Pyramid Head. I want an adorable Pyramid Head amiibo.