What would be a good research topic about excessive exercising?

That depends on what you are looking for. Are you looking for problems caused by excessive exercise or are you looking for how excessive exercise will affect someone with certain conditions? If it is the latter, check out Marfan's Syndrome as people with that disorder are generally pretty restricted in the amount and kind of exercise they can do,

What would be a good research topic about excessive exercising?
Muscular injury.
Reply:Take the angle of addiction. Some people have addictions even to positive things. The psychological side of excessive exercise would be interesting.
Reply:just research the stories of the outcome of those who do, most have been young healthy people who over did it and what was to be for good became fatal.