I expect to see another 3D Zelda in 2021.  2020 will be for Metroid.  

I think the splatoon devs are working on Animal Crossing now. Hence why they are ending or have ended the free dlcs for it.

I agree that there will be a Super Mario Odyssey 2 in 2020. I think we will see a smaller scale Zelda within the next few years, and a BOTW scale Zelda towards the end of Switch's life, probably 2022 or 2023.

Odyssey 2/3D Mario in 2020/2021. However, If there is a DLC announcement in the next direct, there is no 3D Mario until, like, 2023, 2024.

A new Legend of Zelda game happens usually once every 5-6 years... but I think if they take the Majora's Mask Same Engine-New Game approach, I think we could see a new game in like Mid 2021 to Early 2022. Also I think the switch is probably not going to be replaced until 2023 at the earliest and won't be discontinued until early 2025 at the earliest.
I am 95% sure Super Mario Party will get a sequel in 2019-2020, and i even think a 3rd game is, like, 50/50 for a 2021-2022 release.
Mario Kart 9 will probably have a switch release date, but I think that will be at best a Mid 2022 Release.

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.

NES, N64, Gamecube, and Wii all received 2 original (not remakes) Zelda releases.  Actually, Gamecube and Wii shared Twilight Princess (the way that Wii U and Switch share Breath of the Wild), so take your pick which one to credit with 2 releases.  But that still makes 3 out of 6 Nintendo consoles to see 2 original Zelda releases on the same hardware.  Anyway, the fact that Breath of the Wild was available at Switch launch (having been developed originally for Wii U) pretty much guarantees we'll see another new Zelda game on Switch.  I wouldn't expect it before 2022 though.

edit - 2021 is possible too

Why does everyone seem to not know that the Splatoon 2 devs said they'd stop supporting Splatoon 2? They said it a long time ago, it was even announced in a direct I think. After 2018 no more updates. Guess what year it is.

I think we may see an Odyssey sequel sometime next year and perhaps an ARMS 2 to tryand refine the formula established in the first game. I also think a Xenoblade 3 could be a possibility. They already have the assets and foundation laid out from Xenoblade 2 and it would be a good eay to capitalize and build upon the series’ growing popularity on a higher and active install base.

As for Splatoon 3, I don’t think we’ll see that until their next system. We could get a spinoff though, I’m very curious to see what a Splatoon RPG could do.
And while we I am certain we will see a new 2D Zelda on Switch as well as a Skyward Sword remake, I don’t think a sequel to Breath of the Wild or a new game in that formula will come until the launch of Switch 2 or whatever the next system turns out to be. Nintendo is 2/2 with a 1.000 batting average and 2 home runs when they have a new 3D Zelda laanch with their new system. I think they’ll want to continue that trend after seeing the obvious benefits they have reaped for both the Switch and the Zelda series.

2020 at the earliest.