Historical, fantasy, magic, horror, supernatural, survival, vampires, demons, pirates, mutants, ghosts, romance (FxF, MxM, MxF) (Romance should be part of the plot and not the whole plot in itself), etc.
If you only want to store them and not use them for now, then as others have said, you should put it in the blog or in the showcase forum. Once you want to use an idea, you either go to the seeking one x one partner forum or the appropriate group roleplay genre depending on if you want a 1x1 or 3+ people in the game.