Whats funny is I was just thinking about this over the last few days.
The Holiday events in GW, while very fun, are sort of losing their luster because they are exactly the same every year; with the exception of Mad King Thorn getting a new joke or two. It would be really nice for a change of scenery, as well as a change in the overall experience.
Dragon Festival I'm unsure what could really be added. Though considering it was sort of a preview of Nightfall when it originally came out, and since it is, infact, called Dragon Festival, if they were to do something with the GW2 dragons I think that would be a lot of fun. As I suggested in a topic ages ago, lull us into a false sense of boredom with a simple reskin from last years DF, then have the finale be crashed by Primordious.
I'm not ashamed to admit, I'm a bit of a Halloween nut. By far, it is probably my favorite holiday of the year, not only in GW, but especially in real life. However, what I like about it is not the scary jump out and grab you type of stuff that GW features, but the Autumnal atmosphere that permeates the season.
It would be nice if this year, instead of having the NPC's reskinned to the undead creatures of Kryta, they stay the same, and the towns decorations are turned to a late evening featuring golden maple trees and 'cutsie' Halloween decorations; handmade tissue ghosts, hay bales with pumpkins and Jack'o'Lanterns, things like that. Something that is more akin to the atmosphere of trick or treating as a young kid, and less about undead scariness. A few quests would be nice as well. And considering it's been two years, I think enough Tyrian characters have travelled to Cantha to allow Cantha to take part as well. It would also be a nice test for the seasons system in GW2. *hint hint*
Wintersday, I'm not sure what exactly could be done. I strongly support the idea that the winning God could be chosen through an AB type of battle, where donated faction is added up to determine the winner. And It would be very nice if there was a Mini Black Moa type of scavenger hunt for a Mini .. I dunno, Grentch?