Don’t worry if you’re stuck! I’ll be posting some guides before term starts about what the psychology lab can offer you in terms of dissertation support (stats support, equipment and facilities) which will also aim to give you an idea of the types of dissertations this will help you to think up.
In the meantime one suggestion I’d make if you’re stuck for inspiration is to go over the BPS Research Digest with a fine tooth comb, and subscribe to the newsletter! This is a really excellent resource with interesting cutting-edge research which you could think about developing further or taking in a new direction. If you scroll to the bottom of the screen in the BPS Research Digest, the categories allow you to select your area of interest. There are also links to more blogs with lots of ideas. Good luck!