I would definitely like to see boss encounters added for all events, even if they're relatively simple. Boss encounters keep me going back to holiday events that I am otherwise done with, and updating items etc should be relatively minimal effort.
I believe that all holiday bosses should be available to characters of almost any level. For example there could be Frost Lord Ahune for level 15-19, 20-24... Only problem would be the loot which would need multiple versions.
I'd like to see more "world affecting" type stuff. Or at least more going on in the city during certain events. After all the stomping the event bosses have taken, it's about time they start being a bit more pro-active. IDK. Events in WoW seem very "meh" compared to what newer MMOs are doing.
New buff foods and treats. Essentially they give you a temporary replacement to regular food and flasks for level 90 characters. Can be found out in the high-level world zones in gift boxes looted from mobs or spawns in the world. Would show up with a map marker akin to how crashed carts did in Battlefield Barrens
Defending Greatfather Winter; every few hours, Ironforge is attacked by The Greench and his minions. Players must prevent them from destroying presents and capturing Greatfather Winter. Uses HP scaling to account for the number of players and should present a moderate challenge for an un-organised group. First successful completion awards a guaranteed item from a specific set (like 5/6 different things). Further completions have a chance to award the rest of the items, or gold/vp/flasks/etc.
Revamp of the Horseman instance; perhaps we could get a scenario where we witness his creation. First completion should come with a vanity item of some sort.
Improved Horse-man event; much like the BF Barrens/TI stuff, have him do more around Elwynn Forest, rather than just attack Goldshire. Such things could include spawning waves of pumpkin minions, appearing as a tough world boss (with an expanded loot table - more pets/items) and setting fire to different spots, including the Abbey and Logging Camp. Lower levels would gain a buff whilst attacking him that reduces damage taken&increases damage done based on their level to enable them to contribute.
Basically, less "daily quest" and more "this dude is fucking your zones up, go stop him!"
One thing I'd really love to see is some new transmog gear from events. The Christmas tree dagger is nice, but I'd like to see it expanded to be something like GW2; a giant candy cane mace, a star-shaped wand, a snowflake shield, etc. Hell, if Blizz is worried about things being too goofy then put an event timer restriction on them, just some variety and new things to collect would be nice.
I'm not sure, perhaps could make a special tabard for winter veil with like some wrapped present boxes on it. Maybe can change the Greench and make it a instanced event like Ahune? Add some nice loot to him...maybe some weapon with a fire special effect to counter the Ice Staff...
I think it'd be nice if they kept the gear rewards from holidays somewhat relevant in terms of item level etc. and maybe they could add a FUN daily quest/minigame thing to the different holidays so there could be something to do after you have all the achievements. Rare drop mounts or pets are cool but missing even a single day and not getting the drop can feel like a punishment instead of fun.