come up with a robot in response to a problem. that is how all great robot come about. not good but great!
there is no point to build a robot that reads the time out allowed and call it ground breaking, any robotics hobbyist with average programming skills and the right hardware can do that overnight.
a great project would be a ground breaking project, something original not been done before. keep this in mind that a project like this is incredibly difficult, expensive and not to mention time consuming therefore it would not be good or practical for a science fair project.
a good science fair project will have aspects of science incorporated in the design. will be relativity new and show a good amount of work for the appropriate time spent. it doesn't have to be an original concept, it could be further development in the hardware used or the way in which you approach the programming side that is what you focus on.
if you want come back with some ideas about what you could possibly do. give us the time frame that you have, and some other details like age or year level so we can estimate the standard/ or amount needed.
and remember there people here on the forum are always willing to help to extend and further people robotics ventures
good luck with the brain storming