My guess is that it will be something like a larger screen Switch (like 3DS XL).
I really don't expect any changes to Framerate, Resolution, or Operating System; rather, any SoC changes would go to support an even longer battery life and possibly allow for removing the cooling fan and vents all together.
Along with that, I think they might change the exterior design a little to feel a little more seamless, as far as appearance and feel are concerned.
Other possible additions to the Nintendo Switch family could be: A Cheaper TV Only Switch (Like the Vita TV) or A Cheap Clamshell Switch Lite (Like GBA SP). Though, I think those would be more suitable for end of life low price models.
A higher performance Nintendo Switch could happen Fall 2021, but with the Switch user base close to 100m by then, it really wouldn't make any sense to split the user base with any exclusive games.
Then it would just be a higher priced revision for enthusiasts.
I just don't think Nintendo is going to aim at going that far up market to then be compared against the PS5 and XB4.
