Amore challenging (but hopefully rewarding) idea is to start (or continue) reverse engineering a game where no source is available. Two good examples are the pair of students who drastically improved ResidualVM's support for Escape from Monkey Island, and the work on improving [[Operation Stealth]]. Another option is to work on merging someone else's reverse engineering work, such as was done with the [[ZVision]] engine.
If you'd prefer to improve ScummVM more directly, there are even more options available there; in the past, students have improved our OpenGL support, added a testing framework, written a new GUI framework, improved sound format support and added support for high-colour (16bpp and above) graphics. It's difficult for us to imagine ScummVM as it was before some of these projects, you can make a huge difference!