I expect the new model will allow us to play "docked mode" undocked. As almost every single Switch game features docked mode settings, the new model will just set itself to "docked" when you are playing in portable mode. Alternatively, users could be able to select "save battery", which would play the games in original handheld mode and prolong play-time by a few hours. This solution strikes me as very simple and easy for the users, while adding lots of value to the product without Nintendo having to put much work into; exactly something Nintendo would do.
For docked mode, I expect some sort of additional "power mode". Maybe automatic upscaling for games that haven't been optimized for the new hardware. But if devs want, they can make use of the extra power to optimize their games (instead of just an automatic resolution bump or smoother frame rate).
Probably 64GB of internal storage, as this is most likely as cheap and widely available as 32GB was back in 2017.
Basically, I don't expect Nintendo to push any boundaries. That's their philosophy: They are not going to cram the most impressive CPU into that thing, just grab what is already available and cheap and safe to implement, while adding value to the product. Same happened with the Gameboy Color, the DSi and the New 3DS: Nintendo didn't design these consoles with a mindset of "let's make this powerful". Instead, they looked at the state of electronics and said: "X years have passed since the original product. This is the tried and proven technology that we can implement into the new model without increasing costs." If screens have progressed to make 1080p screens as cheap as 720p screens were in 2017, so be it. If there's an X2 processor that's as cheap as the X1 was in 2017, let's put it in. If 64GB of storage is as cheap as 32GB were in 2017, why not include that? That's basically how Nintendo thinks.
