If you're doing it for fun, why not do a personal side project rather than being paid to work on someone else's idea?
I didn't like any of the other RSS readers out there, so I wrote my own, and now I'm using it and liking it. (I was pretty annoyed when Google Reader shut down.) Now I'm working on my delicious clone. (I really liked that site and was sad when Yahoo and AVOS ruined it.)