Heh, I loved the engine test, but didn't try the rat one. Then again, I'm rather a sucker for shooting zombies.
Honestly, what I've always wanted to try is writing the deepest, most literary story that I could possibly write, and putting that into a visually expressive but simplistic 2D game. Think something along the lines of the first and second GTA games, or maybe that old Mageslayer gauntlet clone by Raven, or that other game that used the same engine, I forget the name. But basically: just fully embrace the mediocrity of the technology and then glaze it with layer upon layer of story and character as you cook it. The joys wouldn't be graphical (though again, I think you can do much stylistically if you plan carefully), but would be entirely in the realms of fiction and mechanics. That's what I'd love to do. Keep sizes small, don't throw in voice acting or whatever (or keep it minimal), and just instead focus on what you know your strengths to be. I always want things like that for my PSP and such.