Since this community seems to include a lot of Ph.D. students, and because there is a provision for attracting even more, I think this question is very appealing:
What topics do you suggest for Ph.D. students to consider for their thesis?
I intentionally didn't restrict the topic to any field in TCS.
Obviously, very broad subjects of study, such as "logic" or "complexity theory" are not adequate answers.
On the other hand, ideas like "combine logic X with Y algebra to solve problem Z" are completely acceptable. (Since this is not MathOverflow, "problem Z" should have some TCS flavor.)
The more you elaborate, the better!