Honestly--and maybe I'll be wrong in a year's time--I think this is very unlikely, if you're referring to a brand new IP. It took Inklings three years after their debut to get in, and all Splatoon got in Smash 4 was a Mii costume and a trophy. I don't think it's unreasonable to suggest that a new IP would get a similar kind of representation, but if the reaction Corrin (coming from an already-established franchise in Smash) was a precedent (and we know for certain that there are limited DLC 'slots' this time), then people would lash out and despise them. Plus, including someone from something that might not even pan out to be a critical or commercial success would just be a bizarre business strategy, even from the advertising angle.
It certainly came off like I was guessing a new IP, but Splatoon was just an example to avoid putting a number at the end of it. I meant a new
game in general. Something like, say, Xenoblade Chronicles 3, or that rumored Star Fox Grand Prix game. A game published by Nintendo that hasn't been announced yet at all, but has a new character they plugged into Ultimate for dual-promotion.
There's a difference between not being sure if a game will pan out and not being sure
how much a game will sell, though. For instance, take their Pokemon reps. There's no telling if Greninja would flop as a pokemon or if that generation of Pokemon games would undersell, but
Pokemon itself is a pretty hard guarantee of a multimillion seller. If Nintendo has such a guarantee up their sleeves for the 2019 Christmas season, a Smash fighter will A) likewise be guaranteed a significant amount of sales, and B) further promote their winter hit to outside audiences by featuring in one of Nintendo's other big games in advance.
The timing is simply right
if Nintendo continues their recent trend of holding off major winter blockbuster trailers for the E3 prior.