I kinda miss the seasonal bosses that WoW had, like the Greench and Coren Direbrew. I wouldn't mind if they had a trial that returned each year that you can get new rewards each time it comes back. Can have like a level 50 version and a max level hard version.

As for people crying about your idea being too much work...well it's why we now have events where all you do is talk to a couple NPCs. They complain about the game being too much work then complain about the game being dumbed down.