I'm not asking you to come up with an exact subject for me. I'm asking for some kind of guidance.
I know that Google tests the knowledge of graph theory quite strongly. Maybe it would be a good idea to write on something related to this topic?
I'm a computer science student.
A Masters/PhD thesis should be in something you're interested in and deeply care about. More than getting a job, it should be about advancing the frontiers of available knowledge in a given field.
If you do really well employers would line up to hire you.