Thesis for what? Bachelor?

I would recommend pick something you like and have some previous knowledge in. If you know computer graphics, that's lot of advanced things you could do with it. Other option is something related to game AI. Something related to project management is also possible (you could even take PARPG use of SCRUM as study case)...

Anyway, depend on your likes. I would recomend you searching some papers (ACM - http://www.acm.org - is a good resource of them) and seek for something of your interest.

AI isn't really my main interest back on my University times, but doing a quick search now I found some papers that could be of some use (most of them are for strategy games, but I think you can adapt some ideas to the genre you suit most):

Knowledge acquisition for adaptive game AI - they use the stratagus engine to implement things;

A game AI production shell framework: generating AI opponents for geomorphic-isometric strategy games via modeling of expert player intuition

Hierarchical pathfinding and AI-based learning approach in strategy game design

For CG, a view on previous siggraph (http://www.siggraph.org/) papers and you can find some that applies to games (real time applications)... Also, looking at older non-realtime papers and try to develop it now as a realtime approach could be a good work.

PS: If you couldn't download papers from acm or siggraph and couldn't find it anywhere else, tell me wich ones and I download and send them to you.