I thought the general convention was:

Bachelors - Term Paper
Masters - Thesis
PhD - Dissertation

In any event, the OP seems to either be deliberately vague about or isn't sure what level of degree he is going for, in spite of being asked several times. I also find it odd that he is seeking ideas for a dissertation here. You'd think someone in an graduate degree program would be more influenced by research going on at his institution and being done by his advisor(s) than by asking strangers on an internet board.