Not sure if many of you have seen it, but Arcomage is a really fun card game made by 3DO and featured in Might and Magic.
It's good to play against friends and on the move except it's hard because of the game rules you need to be on the computer.
Basically you need to get your tower to a certain height, get a certain amount of resources or destroy the enemies tower.
I think that this would be easy to make on the PSP and would be worthwhile. Even though it's copyrighted by 3DO (I think) something similar could be made... Just a thought.