I don't think we need more festivals but rather more variation like changing locations or quests so it's not the same old, same old every year. Maybe there could be a set of two or more quest groups that can be activated ever other year since distance can help making older content look fresh again.
As a fluff lover I will welcome new cosmetics for my character or her pony but especially go wild over new emotes (dances, emotes while sitting, riding or walking....).
As for shorter events like the treasure hunt I would like to see something like a region- or even Eriador-wide scavenger hunt, maybe finding objects or even NPCs not unlike the members of the Inn League and doing something for them before we can move to the next point (ideally randomized like the riddles of the Inn of the Forsaken instance so there might be some challenge left ).
I remember that before the european servers were closed the GMs did some events in the open world. Those were fun - at least until the slideshow started and the servers went belly up. The deadly lag from so many characters in one spot and all the fighting animations and effects are probably the main thing that hinder a repeat of such an event but otherwise I would love to see something like this again.
[B][COLOR="#FF8C00"]New Order, maggots: Eat more carrots! The Dark Lord isn't pleased that none of you can see further than the end of your sword.[/COLOR][/B]