Ask HN: Please, help me to choose a GPS app to develop

During my Winter-Spring 2011 Term at the university I have to develop a GPS application (Lin&Win) that helps a user to play geocaching (it must take coordinates in a LOC file and guide a user to find all hidden geocaches).

Since I have to spend my time on it anyway, I want to develop some additional features that will make it useful to the real users. And probably I will publish it under GPL.

So the question is: what GPS application to develop in these 3-4 months?

The most obvious answer is an Android app, but I think there is a lot of geocaching apps already, and new features needed instead.

Or maybe there is a piece of GPS software that has no open source alternatives? I would gladly consider that.

Any ideas?

Thanks!