This is sort of a late reply to the boggart idea
I always imagined that if Luna faced a Boggart before meeting Harry and the rest, the Boggart would sort of - form an image in her mind - kind of like the part in the books where Snape delves into Harry's mind during occlumency lessons - or maybe like a movie roll ... you know?
I imagined thoughts like being surrounded by people (hundreds) and not being able to speak/voice her opinions, or maybe she would see everyone around her hurt or dead - because Luna seems to be the type of person that would care about everyone, she wouldn't make prejudices. Something like that, where either her freedom is taken away from her or an image of someone harmed in some sort of way.
But maybe that is just how I think, anyway. :cool: