The ability to change the combo, hiterror, and other objects' opacity. This would be amazing for mania, as having very bright objects in the middle of the play-field can be hard on the eyes. It would also make objects you usually place on the outer edges of the screen less noticeable if you're not looking at them directly, making them less distracting.
Sorry if this has already been suggested, I don't know how to search through suggestions to check.
Sorry if this has already been suggested, I don't know how to search through suggestions to check.