You are free to pick any project you want as long as it has a computer graphics related component in it. It can be some pet project you are working on at your free time, or something left unfinished in another course. For example, you can have projects from the CG course and CG seminar that you want to do additional work on. Project can also be related to your thesis work, if it includes a large development component.
If you do not have a pre-existing topic, then read on. Following is a list of sources, where you can find a good project topic.
APT Game Generator
This is a community of game developers, who are working on a number of game projects. You can participate in any of their projects or if you want to work on a new game, then you can propose your own idea and join the community. Meetings take place every Monday at 18:00 in Aparaaditehas.
CG course projects.
In the CG course there were several proposed project topics with practical implications, which were not chosen. Furthermore some of the projects that were chosen can be continued. This means that you can contact the person responsible for some topic and propose further development.