I don't have any thoughts on a big gimmick for the system, I'd rather let Nintendo surprise me on that front. But there are some smaller details I'd like to see.
The most important thing I want from the Switch 2 is the ability to run Switch 1 titles at a higher clockspeed. Don't downclock like New 3DS did. That may not fix every bad port, but hopefully it'll help with some of them. My concern is that most titles that should be running on Switch 2 have already been ported to Switch 1, poorly, and won't get re-ported. Instead we'll get ports of newer games that are again too much for Switch 2 to handle. So I just want the back catalog to run nicely and I can go back and play those games.
Variable refresh rate could go a long way. Not because I expect games to support high frame rates on Switch, but because this can also help low framerates feel better. Or even if not variable, just supporting 40Hz could be good, Steam Deck has this and it's a solid middle ground for titles that can hit it.
Also, give me Hall Effect analog sticks. I remember when 'Nintendium' was the meme to describe how durable Nintendo products were, I don't know what happened to the build quality this generation but it's atrocious how bad Joycons are.
This might be too niche to be worthwhile, but it could be cool if JoyCon2s could be mounted vertically for shmups and DS games on NSO. The DS library is full of classics that are semi-inaccessible just because most hardware you could try to emulate them on won't actually be an ideal experience. But that's kind of a fairly limited use case to justify a feature like this, so I doubt it'd happen.