it would be cool to see more holiday bosses, because it's something limited and exciting, though i understand lore contraints with that. alternatively it would be fun to see random stuff in the winter veil presents, kind of like the bags that drop from warbringers and mop rares, but level appropriate.
Holiday costumes for vanity pets! I want to run around with a ghostly skull wearing rabbit ears! Or a red panda wearing a Santa hat the possibilities are endless!
I'd love a new holiday event of some kind... I've been done with all holiday achieves for years now. Aside from random pets or a quick achievement one shot, I've rarely given them any thought or attention for just that reason. It would be nice to feel excited about a holiday anymore. Even the gear from holiday bosses hasn't been worth the trouble to spam dungeon queues after.