If forced, Nathan would just go with some stupidly dark sunglasses and a trench coat, possibly leather.
Camelia might get herself a witch costume with broom and everything and fake flying with her semblance.
Anna would go as a ninja. Most definitely.
Brock would be a pirate.
Helena would probably go as a sort of wild ranger and bring her critters along.
Maena would be an Atlasian Air Force general.
Mark would, uh, be a postman I guess?
And that's all my approved chars.
Nathan wouldn't go