Fixed, but bulky, something that limits speech, creates an overwhelming lisp. Although the shluuurp of removable appliances definitely also appeals. Love headgear too! I love the idea of something prominent, noticeable, and impinging. Something that causes an adorable embarrassment. Bondage of the submissive, if you will.
Expanders top/bottom complete with rake/tongue guard, coloured lip bumper, headgear.
Favourite ligature colour has to be 'hot pink'.