I would like to think I would create something for the greater good but if I was able to do one thing it would probably be a way to make myself immortal and also stop aging. I can tell right now that it would be terrible.
One way has me being the only human capable of it meaning I have to watch my loved ones all die before me and avoid being "examined" to see if others can duplicate it. Avoiding all that, I would eventually become the equivalent of a Homo Habilis in the modern world due to evolution. If everyone becomes immortal, overpopulation leads to shortage of supplies and space which leads to war and death.