World of Warcraft has a lot of holidays, but some are rather ... bland. Take the Harvest Festival, for example. No, really, take it back in its current form. I did it back when it was first released on several alts, and I don't think I've noticed any real changes since then. Let us do more Peacebloom vs. Ghouls to save the harvest, or let us go to Caverns of Time to meet the fallen ... um, heroes we're celebrating. Give us a new mini-feast perma-item to lay out for the event, similar to the celebration mug that Brewfest almost always got.
What about Day of the Dead? It's got a few fun things in there, but remember when we first heard about it via datamining and thought it was the return of the zombie apocalypse? What if Blizzard actually brought that back, just for a few days a year? I know people who don't play WoW anymore who'd resub just to get in on that.
Hallow's End got a facelift, so which other events should be considered? Is New Year's not new enough? Does Love is in the Air need some more love? Is there not enough "Yarr" in Pirate Day?