I think they holiday events are all pretty great and they're all varied from eachother which is great. There's always some minor stuff they could tweak but overall they're great.
I would like to see more holiday currency and something interesting to buy with it, like pets. Pets purchased with holiday currency would not be tradable, keeping the holiday from being a money grab. I like the idea of being able to put more time into earning a holiday pet vs a daily shot at the RNG. The valentines event is a good example, really like the way they execute the events.
Some additional holiday themed content like pets, mounts and mini quests might be interesting. Things haven't changed much in the last 2-3 years. Also, there should be more things for people with lover level characters to do. The seasonal bosses are always reserved for people at or very near the level cap, so it would be nice to give others something to play around with in the meantime and possibly give them a healthy XP bump.
I would like to join a dungeon on every holiday and not just on Hallow's End / Love is in the air / Midsummer / Brewfest. And completing each dungeon you could get a great loot like mounts, pets, and vanity items!