I'm not a black belt yet, but this is what I've been told about the thesis:
In your martial arts journey, you will find one or two things that really stand out as important to you. My instructor did his thesis on Broken Rhythm in fighting (moving unexpectedly so your opponent doesn't know what the heck you're going to do next). Another instructor did his on economy of motion, or causing the most amount of damage with the least amount of effort and movement. Each of them picked something that stood out as really important to them.
I'm going to do my thesis on women in the martial arts, and why techniques need to be adapted to work for smaller, weaker people. This has been really important for me, because as a female, when I teach, I try to give the females in my class the ability to make the techniques work for them. I remind them that for a block to be effective, it has to be of a force equal or greater than the force of the strike coming at them. Honestly, how many ladies can produce enough force to stop a punch coming from a 200 pound, well muscled man? not many. However, if you alter that to a parry and an angle change, the strike is deflected and you're out of the way. Then, you either have the opponent's groin right in front of you and wide open, or you've got their back to you, and there's a lot of damage you can inflict there.
I'm going to expand on these theories for my thesis, pick several techniques to alter, and go from there.
hope that helps.