i'm struggling with the same problem, i'll be doing my last year after this sabbatical year, and the subject of thesis topic is on my mind. i'd suggest you pick a site and theme carefully. over here its usual to pick several areas of interest, investigate them all then make an informed decision to develop.
i'd advise you pick a site you know well, with good reasons for your choice. dont pick a field beside your house because its an easy survey. look around you. where should things be built? what areas need development and rejuvenation? where could your work make a positive impact? because when it comes down to it that should be the basis on which we accept projects, and the generator for our designs.
keep us posted on where you go with this. its interesting to see a budding architect make these decisions. because in the real world they must be made almost on a daily basis