The main issue here is that there is often too much information available: too much information kills the information.
Fortunately, this problem being not restricted to students seeking for a dissertation subject, large security companies invest time and efforts to provide a clear overview of:
- Current trends in the IT security world (both from the offensive and defensive sides)
- Perspective over the past activity,
- Emerging technologies and challenges,
- Links to main facts and events anlysis.
This seems like goldmine for such need. Even better, such report are freely available on the Internet and can be found by searching for terms such as:
- cyber threats predictions
- emerging cyber threats
- cyber threats forecast
From there, I think that any student should be able to find a subject both motivating and suitable for his assignment, plus some argumentation elements to show to the professors why the chosen subject is relevant.