Ambition's always a good thing, especially when it comes to learning. My first project, the one I'm working on now, is a Mario & Sonic platformer combining both series(es?) and everything in them. Just think of taking the default Platform behavior you're working with and trying to tweak it to feel exactly like Super Mario World-Mario -- let alone Sonic. It ain't easy. <img src="smileys/smiley5.gif" border="0" align="middle" />
EDIT: Oh yeah, I forgot my point: I learned a lot more shooting for my dream game than I would've if I stuck with some really lame generic platformer that I was originally gonna do. <img src="smileys/smiley4.gif" border="0" align="middle" />