I don't know about Splatoon 2. Wasn't the feeling already that 2 was very similar to 1. I doubt they'd put another one on the Switch unless they really change up some stuff. I don't see it coming but if it does I think they'd wait a couple more years, definitely not anytime soon.
Definitely another 3D Mario will be coming. Not sure if it'd be a direct sequel to Odyssey though. I would say 2020 or 2021.
I think they'll probably do another open world Zelda which could be called a BotW sequel though I'm sure they'll change up a bunch of stuff, more like a spiritual successor. I'd say 2021.
They will undoubtedly have one or two more Mario Party games on Switch.