Although I'm completely confident they won't announce Switch 2 until next year, the software lineup is more personal prediction, but here's my take. I put the most stock in a new 3D Mario game. The question is, what do they pair with it. Personally, I don't think they will re-package TOTK that fast. I think if they do "TOTK DX" it will be later on to fill a gap. This is assuming Switch 2 has BC. If it doesn't have BC, then TOTK DX will be there Day One.
It's very feasible Switch 2 will be BC and the launch will be Mario and Metroid. While Metroid might not seem big enough for launch, on the contrary, this could be the launch to make Metroid a big game if they knock it out of the park. Especially if they kill it with a multiplayer mode. It could almost make Metroid a household name if executed correctly. They smartly led into this by porting Prime 1 with a modern control scheme.
I think we will see only one face of Mario at launch, that means the one new 3D game, and no Mario Kart or 2D title. If it's BC, and the games they deliver are hype, it won't matter. At launch they will sell everything they can make no matter what. And then keep the momentum up by spacing the big guns apart. We aren't going to have 3D Mario, MK9, TOTK, Metroid etc all at the same time at launch like I see some muse in other threads lol. Just because they have it loaded doesn't mean they're gonna fire it yet. They are masters of spacing and timing.
As for Smash. Worst case scenario is a port of Ultimate that runs or looks nicer. But there will be Smash. Be it a port, or a new game without Sakurai, or if he gets dragged back in. How many times have tired Japanese men said "I'm done working" anyway, we will see.