One suggestion in order to find a hypothesis which a famous scientist has not completed would be to think small. Many of the things which have made a true difference in the scientific world have been large breakthroughs, or smaller breakthroughs which led to larger breakthroughs later.
That said, here are a few suggestions regarding possible hypotheses which have not been studied by a famous scientist.
1. The number of people who speed up at a yellow light instead of slowing down is...(choose if you think more will slow down or speed up).
2. A female panhandler will receive more donations than a male panhandler. (Or, a female panhandler with a child will receive more donations than a male panhandler with a child.) Many other variations of this exist.
3. Men are more likely to open a door for another person than a woman (or vice versa). The addition of child/children can be another experiment.
The link below denotes the 100 most famous scientists throughout history.