What if, while Voldemort attacked Harry in the Ministry his other followers were setting fire to Hogwarts? Half the dorms, the classrooms, and paintings were burned, not even fixable with the most powerful magic. So they fix everything the muggle way. Which takes time. Time that they don't have.
The only dorms left standing are Ravenclaw and Hufflepuff. Therefore they split the Slytherin and Gryfindor among them. What happens when Luna returns to school and finds out Draco will be not only sharing her dorm, but her common room, her classes since he failed the year before, and not only stares at her, but tries to protect her as well.
Can Luna stay clam when his dad is one of the main reasons her father is so paranoid and almost didn't let her come to school this year. And what happens when she gets a little too attatched? What happens when his dorm, his old common room is fixed, and he gets to move up to his year classes only.