You answered the question yourself. Preexistence of similar product is an indication of the demand. Given that your product have some additional feature, you should try to leverage the same and gain first mover advantage. Once you hit the tarmac you'll see other ideas evolving within yourself to maintain that advantage.

Secondly, there's nothing called good or bad idea. What fails an idea is absence of optimum planning and poor execution. Make sure you plan your road ahead into small achievable milestones.

Hope above should be of some help!!