Fyi all the devs hang out in this thread:
So what solo developer game do you want to make? For the purpose of this discussion, assume you are an ace coder and brilliant at art. The game I used to want to make was an SP paranormal investigation game, but this was before Phasmophobia. Now if I made it, it would seem like a copycat game...
For me it's UI—what to include, where to put it, how to access it.
The other biggie is instructions and tutorial—what to include, where to put it, how to access it.