I find the best bet is to make a mod or total conversion for another game, that already has an editor and all the game assets for you to use. If your proof of concept mod is fun to play, and has a lot of players, then porting it to another engine is simply a technical exercise... the gameplay is the important part.
What most people think would be a great game rests on the idea that it 'sounds cool', and little or no planning goes to the actual gameplay. A game of Vampires versus Werewolves sounds cool, but without thinking through in your head how it will play, you are almost guaranteed to make a dud. Took me a year to make a mod for Tribes (the original), adding all kinds of things like Medics that could heal (far before AAA titles had them), but I designed it as I coded it, and in the end it flopped.
Started hating programming after I had a job doing it, so haven't been doing much, but these low cost engines are enticing me to start again.