Figure out what technologies you want to learn, and you'll narrow down designs.

- Graphics tech: simple 2D, animated 2D sprites, simple 3D, rich 3D? Static scenes, or performance intensive graphics with fluid framerates?
- Sound tech? Mixing, surround, etc.
- Networking: leaderboards, social media integration, realtime client/server multiplayer, realtime ad hoc multiplayer?
- Input: acceleration / tilt sensors? Multitouch?

Be wary of going for any classic action game like Tetris. The available input methods are bad for that. Making an action game work well on mobile requires "different" designs which hinge on timing and decisionmaking instead of continuous input. Racing games where you can use sensor input are about the only classic genre that should work.

Incidentally: what's the state of the art in mobile racing games using sensor input? I'd like recommendations of specific titles.

A game where you need to move the phone around a lot to make sure the player will seem like a total idiot if playing in public.

Holey Hanna is a possible name for a game that I thought of awhile back. Holey Hanna is a planet full of holes like swiss cheese. The interesting part is interacting with the gravity of such a world. For instance many people probably don't realize that there would be zero gravity at the center of a density consistent spherical planet. Tilting the phone could be your flight controls, but much of your acceleration is achieved by how well you navigate the gravitational field. A small solar system like ours where the planets are shot full of holes would really make for an interesting yet educational game. Anyone familiar with an existing game like this?

A deposit of Plutonium, Gold or Lead would produce a large gravitational effect. Gravity is how real astronauts slingshot their spaceships to the moon and back. Tunnels through a planet would cause a natural oscillation like swinging on a swingset, so timing your rocket bursts (or gun fire = laws of momentum) would be key to saving fuel, etc. Lazers could cut holes with less impact on ones momentum. My programming experience is limited to scripting languages. Is there a good game engine that models gravity?