All I want is for Nintendo to take the best aspects of the 3DS and make them better.
This is a close approximation to what I would like. One large screen, with a clam-shell design to protect it.
A few ideal changes:
Change the position of speakers so that they aren't covered by hands
Add a DPAD and 2nd analog stick (the second analog stick is a must)
Do something more elegant with that central real-estate. I don't want dual-screens again, but would gladly take them over an obnoxious 'amiibo port'.
Wirelessly charged stylus?
+ Add scroll-wheel shoulders
Then I'm mostly happy. Personally I don't care for 3D and would not mind seeing it dropped, but it's one of the few features that Nintendo can leverage against smartphone gaming - arguably their biggest competitor in the handheld/mobile space - so I can see a point in retaining it.