RC lawnmower is cute, but if you were looking for innovation, that would be more like autonomous lawnmower.
I have some products I will make, but dont mind sharing.
first, an autonomous metal detector. There is alot of use for one of those. Esp if you can get the cost below 100$.
for the hobbyist, you would be taking out the pain in the butt part. The idea specifically is it search, identify, and mark, using available gps, and metal detecting technologies. Deploy such a robot in the park, and find metal trash prolly.
On a more serious note, cheap autonomous robots could be deployed to save lives. In nations where hundreds of kids die yearly from forgotten land mines, inexpensive metal detecting robots could be used to save lives. There are other uses, as well, for the creative entrepreneur.
Remote sensing is a big deal right now. Depends though what you want to sense. I'd write more, but I have to go work.