Literally, anyone can pitch ideas but if you mean write scenes then I would recommend you read up on all the threads in the Submissions board.
If it's purely posting ideas, knock yourself out in the "Content You'd Like To See" thread and post all your fat ideas over there. Just don't be surprised if they're not listened to -- most writers rarely go to the public for ideas as many of us already have very solid ideas for what we're doing/going to do.