What I would quite like to see, is a GBFinal/Deluxe/Whatever that plays GB, GBC & GBA, has a backlight, a charger and one that will use GB cables, GBA cables, and has a built in wireless adapter. I don't like having to use my old GBAs for trading on R/S/E, and even less like trying to use my GBs for trading between R/B/Y.
I would also like to see a dual power source. Using batteries and a charger as would prolong battery life, and stop me from draining the car battery when I'm camping (whoops). Though I really don't think a pair of AA bateries would fit in that thing...
Just my opinion mind you.