Zelda is good, albeit, grossly over rated and is available for Wii U.
Arms is decent. Nothing groundbreaking though.
Mario Kart is Mario Kart. Great game but the same as the Wii U one for the most part.
I haven't played Splatoon 2 yet.
I would say at this specific moment you aren't missing anything.
At this time the Switch doesn't have any exclusive that would make it a must buy right now.
I think you can wait until Mario comes out to cop with no problem at all.
*Unless you don't have a Wii U already, then there are a few more games worth checking out right now, like Zelda and Mario Kart.
And yes, mobility is cool, but it doesn't magically make a game better like some would suggest. Whether you play the game on a swing at the park or in your living room, the game itself is exactly the same, so I don't buy the whole "this version is so much better because I can play it on the go" mantra.