I sincerely hope that Nintendo understands how important the amazing year of 2017 was for the overall momentum of the Switch and plans to repeat that for the successor. I believe there are a couple of key kind of games they need to do that and a few additional important points they want to hit on top of those.
1. An online game.
When the original Switch launched, the online play on it was free. Nintendo Switch Online launched only after a year, in 2018. NSO currently has a decent amount of subscribers, and I believe Nintendo wants to keep it so. Thus it is very important to have a product that incentivizes people to keep their NSO subscription early on in the life span of the successor.
Splatoon 4 seems a bit too soon, Mario Kart 9 is certainly a strong contender, Smash seems unlikely, new IP would be interesting but risky.
2. "AAA" Nintendo game that appeals to a large audience
I'm talking about the kind of Nintendo game that sells in the tens of millions with ease. Zelda is out of the race here, thanks to TotK. It's just too close. 3D Mario is probably the most likely option. Something like Animal Crossing could work too, but for some reason it just doesn't feel like a launch year game to me.
3. A core game(s) for the enthusiasts.
It's important to have something for this audience, as they drive a lot of the narrative around the console on the internet.
A Monolith Soft game and/or Metroid Prime 4(cross-gen?) seem the most likely candidates. I know, Future Redeemed is only a year old, but the main game itself, Xenoblade 3, released in the summer of 2022. Assuming it would launch late 2025, that would be three and a half years of development time, and Monolith Soft works fast.
4. New small-to-AA IP like what ARMS was to the Switch.
Even if said game flops, I believe it's important to have something that makes the platform feel fresh and young.
5. Late and future third party ports that could not run on the Switch. The "Skyrim moment" for the successor if you will.
Games like Baldurs Gate 3 and Elden Ring come to mind.
6. On top of these, if I were Nintendo, I'd be courting Capcom to release Monster Hunter Wilds on the successor, no matter the cost. The release date for the game seems to somewhat match up with the release of the successor, and having a Monster Hunter game as a launch title would be huge for the eastern audience and overall momentum of the system in the region.