Find a partner that can do the front end work. That way you can work together to get to a minimal first version quickly. The longer it takes to get from starting to an actual first version you can sell, the more likely you will run out of steam and give up. Then work together to come up with a simple idea that can be coded within just a few months of spare time.

No one can tell you what to build because if we knew of a simple but great idea then we would do it ourselves!