A lot of people seem to have issues with finding ideas for the theme. I was stuck on it a bit too, and I think my idea is still pretty basic and predictable. A bunch of people die at the beginning of the game, and the actual game is them navigating the underworld trying to find a way to come back to life. Feel free to use this too, if it helps, since I don't think we'd end up with similar results anyway. For one, I'm using RPG Maker software, so unless you're a special unicorn that doesn't do coding like me, then the esthetics will be fundamentally different from the start. Then, the description I just gave is super general, and the specifics in interpretation of the premise alone are going to differ. So much more so the further you go into the game. Not to mention, the mechanics are going to make the whole thing different. I see people exploring the theme with 3D shooters, platformers, top-down view gameplay, etc.
What I'm saying is, just take any idea you like, even if it's not initially your own, and change some aspects about it to make it your own. There are too many variables for anyone to turn up with the same thing as the next person, so don't worry too much about being original, because you'll be unique anyway.
Anyway, I hope my rant was inspiring enough to get you going :) Good luck!