There is likely one planned for 2020 which can be expected to be a year without mainline game. Mystery Dungeon has been the most successful spinoff series, so that's my pick.
There's only one Nintendo console now, so fewer games are needed. I expect Pokémon to become similar to Zelda where it's usually one release per year if everything is counted between mainline and spinoff games.
Really? Pretty much all spinoffs are made by external developers that have licensed the IP, I'd be surprised if the unified platform means less games overall. If anything, I would expect the opposite, now a single release can get a much bigger audience. The only way for this to happen is a massive migration to mobile, which is sadly very posible.
You know it deserves the GOTY.