Something you're familiar with. And at a level required. (A CS final year project might be a language compiler.)
I'm a pretty experienced developer (46 years) and I still have no ideas for an Android app that hasn't already been done 20 times, because I have no current need for an app to do anything that my phone doesn't already do, so it's all a pretty personal thing. What are you interested in that's not too trivial, what are you capable of. I think discussing it with your advisor would be a better way of getting an answer than asking here would be.