There is a lot they could do with it. RTS type gameplay, improving RPG controls, and allowing skills to be changed quickly in games with lots of them. However, I think a lot of the best uses would be in the types of games Nintendo doesn't often make. ZombiU, as the OP mentioned, for example. Metroid, though, could be a good title to really show off the game-pad.
There is a lot of potential but I'm not sure it's going to be reached. It's a 'chicken and egg' situation, kind of. The best games for the Wii U would be ones made specifically to use the game-pad features but no one is going to do that unless the Wii U gets very popular. Kind of a shame to think that it might never be utilized fully.