Are you talking about a dissertation? Or else, its slightly unclear what you want. Also, if it is you dissertation, the themes (topics, I presume) usually emerge from a combination of:
1. Area or niche of personal interest
2. Current research within the area, if possible along a specific angle.
3. Underdeveloped territories within this
4. Availability of some guidance for you to take this all forward.
Once you've done that, you could of course benefit from two way discussions with people in the similar field.