Why not combine MMO and the cell phone? Ok, maybe not MMO in the traditional sense, but something where multiple users could participate in a single game over the network.
A buddy of mine and I were contacted a few years ago to create a true multi-player chess game for cell phones (we essentially offered our services up for free on gamasutra.com for the experience). Players would be able to play chess against opponents anywhere in the world – all on the cell phone. It was a cool idea. I may be showing my ignorance here, but perhaps this concept could be extended to create a game with an endless user base (as opposed to 2 player).
I remember how much of a shock it was trying to develop for cell phones after coming from the MS world of development. There were so many different flavors of cell phones and so many version of Java. The IDE’s were not that great and the debuggers were even worse. It was really a hard time to develop. But now, and especially with iPhone, things appear to be much better. I have actually been creating a little app with Google’s Android SDK for cellphones. Things have improved greatly!