I'd like to see Rumble implemented as a standard feature in the next handheld, it's about time. Closer touch screens would be nice, however, I don't necessarily see the point in dual touch screens. It might get a tad confusing.
Trigger Shoulder Buttons would be great (like on GCN) and an analogue stick would be great. How it could be done is that it can be spring loaded, so when you want to close it, all you'd have to do is just press it in.
I wouldn't mind seeing 3 degrees of tilt (a la Nunchuk) as well as IR/Bluetooth support. Oh, and an SD Slot would be fantastic. They could even port the Wii Channels.
Bigger Resolution is necessary, do you think it would be possible to make it at 480p?