First off I'm not out right complaining about the seasonal events so far as I've enjoyed them and their cheery brand of wimsy, but I'd like to see a little more depth to them in the future.
More then the current re-skinning of the city states and seasonal gear to collect, and the FATE grinds to collect items.

How about extended side-quest chains (across different Levels and regions like the FFXIII event) and a special seasonal dungeon relating to that quest chain and themed to the season/event, in addition to the costumes, FATES and re-decorating we have so far.

Now that most of the major bugs are worked out of the game the dev team might be able to spend a little more time/thought on these things.