Let's GO releases later this Fall so I feel next year's theme ought to be based around it. Namely there will be two to as many as four teams. If it's based on two, then it's Team Pikachu and the other is Team Eevee. If we increase it to four like last time, then the other two teams should be Sandshrew(Kanto or Alolan) and Vulpix(Kanto or Alolan) seeing as they're version exclusives and both would be Pokemon people would start their journey with(and if I'm not sorted into Team Vulpix, there will be hell to pay).
Another theme idea is to base it on the following four types: Fairy, Dark, Psychic and Poison. The mascots for these teams(can be changed mind you) would be Sylveon(Fairy will claim this in a heartbeat), Greninja, Necrozma(didn't people clamor for a Team Necrozma last time?) and Poipole.