Everyone is speculating the obvious choices like Tapu Koko, Lycanroc, Decidueye, and Incineroar.
Sakurai tends to be unpredictable. If I were to guess, I'd go with Golisopod or Type: Null/Silvally. Both are utilized by important characters in the story of Sun and Moon, could bring in very unique movesets and playstyles, and were very well received Pokemon with a lot of fans.(Though this likely didn't play part in the decision behind the potential Pokemon rep)
I'm personally hoping for Golisopod, simply because it's super fitting for the roster. It could be a heavyweight Pokemon, which only Charizard is currently a heavyweight fighter for the Pokemon series. It's design is based on Samurais, and has a moveset that could easily translate over to Smash. Aqua Jet could be it's up special, First Impression could be a close range side special with a stunning effect, Sucker Punch could be turned into a counter for it's down special, and it's standard special could be Pin Missile as a projectile, or it could even take Charizard's old down special being Rock Smash. Obvious final smash would be the Bug-type Z-Move, Savage Spin-Out.