I'd love to see all the Holiday bosses to become queue-able, with major revamps for certain holidays, like adding bosses too. Add a gear catch-up mechanism for alts.
Instead of some holiday events giving low item level gear with stats, they should give cosmetic gear specifically themed to the holiday, similar to what Guild Wars 2 does with their cosmetic skins. Blizz could make it more like the Darkmoon Faire, where you can't get a full set available the first time the event is around. In theory, it might take you several Winterveil's to get the mount, pet, and gear, etc that you want. I think that would be cool as it would add some longevity to the content, giving us a reason to participate every year.
I'd like to see more dungeon bosses for holiday events, as well as keeping the rewards current for the xpac/patch. With the level cap going to 100, the rewards should be a useful ilvl for the fresh 100s. Other aesthetic changes could be fun too, such as cities having snow or something during winter veil..fireworks..etc.
New rewards such as mounts, vanity items and gear for the players that have already done it before!
And I would like to see Hallows End broom to be a real (permanent) mount.
April Fool's Day holiday would be pretty sick aswell!
No need for new festivals - with at least 1 per month already pretty much neglected. Would love to see even a 1 person "team" working on updating the current festivals and keeping them relevant !
i would just be happy is the holiday season quests / story / lore would be different every year, but somehow still be able to gain the old rewards if someone missed them before, or is a new player