Just some initial thoughts on the starters. I'm going to assume that tutor moves aren't in Sun/Moon at the moment, just like how they've never been in the flagship games of any gen.
On their own, Primarina and Incineroar definitely look viable for VGC. Primarina has a Sylveon vibe going for it with its great special bulk and special attack, and its ability, Liquid Voice, gives it Water STAB on Hyper Voice (is it confirmed whether it also gives the 1.3x boost yet?). I can see it being a good spread-attacker throwing off Hyper Voices and Dazzling Gleams, and Water/Fairy with 80/74/116 defenses gives it good switch-in opportunities. It also gets Perish Song for Perish Trap teams and support options in dual screens and Encore. Honestly, Primarina looks like a solid mon; it's viability will probably depend on whether Tapu Fini, the Water/Fairy Tapu, outclasses it.
Incineroar is looking to be a weird hybrid of Arcanine and Scrafty. It gets Fake Out, Intimidate, decent 95/90/90 defensive stats, and a Fire typing to avoid burns, though the Dark typing makes it neutral to Fairy. It gets Snarl, WoW, Taunt, and U-Turn for support options too. And it can still hit decently hard with that 115 attack backed by Flare Blitz. Sadly, no Knock Off though until move tutors come back. Base 60 speed is also great for Trick Room. Funnily enough, its pre-evolve, Torracat, has base 90 speed, so someone's probably going to meme with Torracat + Eviolite as a poor man's Arcanine with Fake Out.
I don't know how I feel about Decidueye. It honestly looks more like a singles mon, and even then, medicore at best due to that sucky base 70 speed. Spirit Shackle has a single-target Mean Look effect, which might work for Perish Trap. 78/75/100 defenses are pretty shoddy though. I'm not expecting to see this much.