-Target platform: NDS.
-Use stylus as a dedicated control, not a gimmick.
-Easy to pick up and play for couple minutes.
-Gameplay should be audio-oriented (think Patapon, Lumines, etc).
-Same rules for single and multiplayer (think Super Puzzle Fighter).
I've developed commercial games for NDS already, so I more or less understand what I'm tackling. The trick is - I'm gameplay programmer and designer. Usually between me and ugly world of VRAM and the like stood like a guarding angel engine, or at least high level API like SDL or XNA. First on the schedule - write a (very) simple graphics engine that will allow me to StartTopScreen2DMode() and so on.
So far I managed to get devkitPro toolchain to compile and created this awesome base for my game:
PS: Oh yeah, my graphics will be typical 'programmers art', so bear with me. Artists will come after gameplay is done ;).
PPS: Don't you like word 'developing'? I could say it all day. Developing developing developing developingdevelopingdeveloping!