If a new Pokémon is coming out on Switch in 2018 I would be super surprised for it to actually be gen 8.
It may not be, but I would find it a bit odd if they introduced Pokemon to the Switch without going to a new generation, especially since they came out and announced they were developing a brand new core RPG Pokemon game for the Switch this far in advance with no footage of any sort. If there's anything we've been shown since Generation V, Gamefreak does not adhere to the strict schedule we thought they had before. Generation VIII will come when they want it to.
I'd actually be happy to move away from Generation VII pretty quickly, anyway.