Backwards compatibility means that by launch the GP3x has plenty of (quality) software for it already, so...
Hmm... What about USB Host and TV out? Will those be on the unit itself? USB Host I personally think is very muchly so needed (preferably three or four USB host ports: one on top, one on left, one on the right, and one on bottom. Or maybe two/three on top and one on each side. Bottom might not be ideal due to the slideout keyboard though). TVout could be utilized through an ext-style port on the top of the unit, but the TVout cable *HAS* to be delivered with the unit itself. Either no TVout at all (bad!) or TVout 'delivered with the unit' (may it be on the unit itself or through a delivered cable).
The SD card slot has to be somewhere where your hands are positioned all the time, so that proper peripherals can be placed on 'clear' places. I do not like at all how my laptop has the DVD-slide on the LEFT, and two USB ports plus extra monitor on the RIGHT, where people usually place a proper mouse. Placing the DVD slide on the right is much more sensible, obviously.
A firmware-only button which takes you to the 'main menu' of the unit (a bit like the PSP's 'HOME' button, but without the forced-to-exit-to-go-back-to-the-XMB thing) would be ideal, too, to improve multitasking and quickly taking a glance at the battery status and such.
What about other buttons? Currently, many emulators use a gazillion or more button combinations to allow for various features. I know that with a touchscreen and the two vertical bars on both sides of the screen caused by the aspect ratios will certainly allow for plenty of 'button room' already, but some extra buttons won't ever hurt, right, especially if the design and hardware/price (I do not think adding an extra button costs much, right) allows for it!
Personally, I like the GP2x for its 'bulky' buttons, which are big and easy to 'feel'. The PSP has very irritating buttons.
Hmm. Time for some futurethink..
You're playing Sonic (what else) on a Genesis emulator on the 'GP3x'. Suddenly you feel a bit of a stutter in the emulator, so you touch the left black space on the screen, and a menu smoothly appears with a smooth fade-in. You tap the bottom-right corner of the screen, a FPS meter appears in the same smooth fashion, showing you a number ranged from 40 to 50. You find a slider which corresponds do the CPU clock setting on the left side of the screen. crank that up a bit, and the FPS meter goes up, after which you close both the FPS mter and the left-side menu. After having done that, you tap the black bar on the right side of the screen, which turns into a 10x10 grid of blocks each marked with a number and a color which is either grey, green, blue or red. You tap a blue block, the 'main game screen' fades away and in fades a snapshot of a savestate, you hit a red cross, and you go back to the game itself. You double-trap a grey block, and the game automatically saves state into that saveslot. Then you tap the slot you just saved to twice again so that you can give that savestate a name, slide open the keyboard, type "Stage 1-2", hit enter, close the keyboard and get back to playing the game.
Hmm.. Seems futuristic and nice. Good times.
