Your thesis can be any number of options really. Any 'cause' for the fall of Rome can be argued as the leading culprit (and each can be broken down into a multitude of sub groups), whether it be social/cultural/religious disintegration (lax citizenship requirement/benefit, Christianity, etc.), barbarian invasion, economic failure, military failure and the list goes on and on, even including the presence of lead in the Aqueducts and plumbing. Perhaps as a bit of an alternative you might want to establish a thesis arguing that the fall of Rome was inevitable, stemming from the concept that nothing can last forever. Much of western civilization, including the rise and fall of nations after Rome, have been compared to Rome, but we often hear little of Rome being compared to earlier human civilization (other than Greece). Its certainly different, but I wouldn't personally be able to support the thesis because I don't believe the concept of fate, and its also very difficult to 'prove'. Still its a direction that might gain points for 'thinking out of the box'?