I would actually like to see new quest chains for the holiday events. I know it's nice for new people to see all of it in it's glory, but something new would be more engaging.
Also, I would love to see a Holiday event that involved some of the new gameplay mechanics that Blizzard has given us, such as Pet Battles, or even events that incorporate old raids (and not just for achievements, but for an actual questline). Think about it...a Hallow's Eve quest chain that involves going into Kara to get to a new boss added just for the event. The raid could be scaled no differently, but the boss could require 5 people. You need to clear the whole place following the quest chain inside the Instance, and then you spawn the boss after killing Prince at the end, if all the quests have been completed. How cool would that be?
I would love to see holiday events have more randomization to them, and for their to always be new non-meta achievements added each year. Making them somehow tied into the current expansion pack would be interesting as well.
And slightly off topic, but i wish there were more things like darkmoon faire and the fishing tournaments; more once a month things to look forward to that aren't necessarily holidays!
As much as I absolutely love the week+ long holidays in WoW, I think we need some more silly short holidays. Talk Like a Pirate Day is a good example. Silly one-day holidays that give you some fun items to play with, maybe even minipets?
I would love something new for Brewfest instead of the same ram run and dark iron attack every year. I have all four coloured steins, I remember when they gave the same one two years in a row and the outrage was very loud.
Maybe something involving pandaria now because of the Panda Brew.
Update the holidays with Holiday Scenario chains like the Solo Scenarios in Thunder Isle. With good scaling, it could give a level appropriate reward for anyone from level 1 to 100.
I wish Midsummer would be changed so we could get a Ahune pet (like a frosty Lil'Ragnaros) by collecting enough flowers (I don't like rare drops, they only make people extra frustrated since the holiday is already limited in time). Since pets are so popular now, this seems like a good idea.
I would love to see the Magic Broom from Hallow's End being available as a regular mount not only usable during the holiday. It would be an awesome mount for warlocks.