Hi Maxicore and welcome to EHN!
First of all: I am very happy to see a fellow Belgian on this forum
I had a similar situation like you two years ago. I pushed a bit here and there and finally got a thesis where I needed to make a couple of useful tools based on Nmap, Snort and the likes. I learned a lot from that.
In general I can say:
If it's a practical thesis -> do something with a well known open-source tool. Create a couple of Nmap scripts for a specific security problem, or a Metasploit module.
You can always take a look at the Google Summer of Code projects (Google it in case you never heared of it), and see if you can get some inspiration there. Hell, maybe you can combine that program with your thesis and actually get paid
If it's more a write-everything-down thesis, you can talk about hot topics in security. Dissect Stuxnet for example, or write about social engineering. Or mobile security (specific on one OS or not).
Just a couple of thoughts, nothing specific. And be warned: I'm not an expert at any of this at all, merely a beginner. But I think I'm saying: surf around a couple of blogs, and certainly this site, and inspiration should rain down from the heavens
If you have more specific demands on your thesis, feel free to say so. Feel fee to PM me with questions as well. The Belgian world of security is small, so it's fun to know everyone in the game here