On the topic of cartoony characters, I wouldn't get your hopes up for too many. The only third party platformer/mascot I can really see making it in is Crash. Any more would be from Nintendo (Waluigi, Geno, Pokemon etc). It's unfortunate, but platforms just don't have much of a presence nowadays then they did in the past, when hardware limitations weren't very kind and platformers pretty much ruled the industry because they were the best looking games back then. Today, most games try their best to be as epic as possible and the only platformers nowadays come from either Nintendo or indie developers. It's very rare you see a big company make one and when they do, it's usually for a notable IP.
That said, there are plenty of "human" or "mature" characters that have a good shot of making it in. Dante, Sora, Master Chief, Scorpion, Doom Slayer, Phoenix Wright, Lloyd Irving, Heihachi, I could go on and on. The only other third party platformer mascot I can possibly see making it is Rayman, and even then, if he were being planned, I feel like they would have held off the Assassin's Creed and Rabbid costumes for him. You do have indie characters and as cool as it would be to see Shantae, Quote, Hollow Knight and Hat Kid in the game, I feel like Sakurai is saving them for special costumes, similar to Sans and Cuphead, as it is a nod to indie creators, giving their characters a place in Smash even if they aren't fully fleshed out characters. Sorry, but I just can't see indie characters making in, except maybe Shantae, but even then, her chances are slim. If three of the biggest indie characters failed to become a fighter, what chance does any other have?
Overall, I'm prepared for anything to happen. Who knows? Maybe we could be getting Rayman or an indie rep or whatever, but the chances are not very high, especially when you take into account all the other characters who have better chances of making the roster. I'm very confident in Crash's chances because of how well his games have been doing recently, and he's a gaming icon at that, which is more than can be said about most third party platformers mascots. Not trying to tell you not to support other mascots, but just don't get too carried away, that's all.