I have a great idea for a movie. Does it belong in Cafe Society, since it’s about a movie, or MPSIMS, since its, well, just stuff??
I would say it depends on what kind of responses you’re looking for. You might put your OP in Cafe Society if you wanted opinions on your idea from moviegoers or if you wanted some insight on how your idea would match up with other current movies or something like that. And if you just want people to crack wise about it, you can post in MPSIMS.
It could also depend on how you want to phrase the OP. IANAM, but these kind of things aren’t set in stone.
What is your primary purpose? Are you looking for investors? Do you want people to critique your script? Are you hoping to receive lavish praise for your creativity and story-telling prowess?