Look back on your education. What topic did you like the most?
Now, within that topic, try to find a specific thing you could study in more detail.
Find a new angle to this.
Here are some totally weird combinations. They are not intended to be workable, but rather to be fun and get your mind going:
-Can S-FTP be used to fight spam?
-Can a home-router act as a Teredo endpoint in a secure fashion?
-Can cheap home-routers with alternative firmware be tamed to handle load distribution over parallel internet connections?
-How should a router be constructed to allow the administrator to selectively accept UPnP settings from clients?
-Optimizing meshes of auto-configured wireless access points, using peer-to-peer negotiation over the air...
etc.