The two game ideas I've had that I've made any steps towards thinking of making anything but a thought are:
1. JRPG with dating sim elements that drastically and completely change the story, character relations, back stories, dungeons and gameplay based on single small choices. Doing it myself in RPG Maker was far too much of an undertaking and pretty much meant there were just 4 short games that diverged based on a single choice at the beginning and then there were three endings for each. All of them radically different in tone, setting and outcome. I could still make it, but fucking hell is it a lot of work trying to get all of those things to work together, and I'm terrible with combat design in every way, so making the game worth playing on top of the basic ideas of the story is next to impossible without any help.
2. Classic survival horror game that is essentially RE1, in a mansion, survive. Only difference is you're in a group and your job is to make sure you save everyone in your group while fighting the monsters. No continue system, you die you start the game over, automatically erasing your save data the moment you load it. Short as hell and beating the game without saving anyone is ridiculously easy and fast, but you get the worst ending. Saving everyone is hard but rewarding, the no continues make every choice actually matter and people told me they wouldn't play that but fuck those people it isn't for them. Oh yeah, tank controls also.