Games. More specifically: 3rd person RPG games for the iPad. Games are the reason I started programming, but somehow ended up being a web developer.
I think most developers are interested in the engine development (a known quantity) but when it comes to the art assets (3D models, animations, textures, level design) that seems like a giant unknown to me.
I would enjoy working on a rendering engine, but if I had to model characters and animate them, I think I'd release the game 6 weeks before my 97th birthday.
If you have talent in all the required spots to do this, I'd agree with the other poster... so 2 or 3 proof of concepts in the app store to hone your skills.
1. Game 1, simple terrain traversal game. Get used to open-world rendering.
2. Game 2, character-focused game with stats. Get used to modeling and animation.
3. <some more stuff>
4. Game 4, Skyrim for iOS
No you can't create Skyrim iOS edition in your free time, but you could create something much much sillier and simpler right?
Get the ball rolling, the creative juices flowing and a few apps in the respective app stores before quitting and going full-force?
I realize there isn't anything prophetical here; if you are an all-or-nothing type, then that will just be a much harder decision to make and I am hoping you get a chance to make it at some point.