Luna could have been in any of the houses.

She's smart as a Ravenclaw for thinking of flying the threstrals. 

Kind and loyal as a Hufflepuff for saying goodbye to Dobby when no one else could. 

Brave as a Gryffindor for being herself. 

And determined enough to be a Slytherin for always believing in nargles, wrakspurts, blibbering humdingers, and crumple-horned snorkacks.