My idea is to have multiple timed-gated events all over the year. One event starts, the other finishes, and so on. Because I believe it's good for new players or players who completed to update to have some new and short events while they wait for another update. I know it doesn't work with old users, but It'll be beautiful for future users.
I have a lot of new players every day, the timed-gated events will keep their hype up over time.
And... I believe this kind of
non-canon / side content should not be permanent in the game. Yes, I may lose some dev-time, as you say. But at the same time, believe me, once I finally reach and make events all over the year, it will be pretty awesome for new players to have new
extra and thematical content every 1/2 months, not only that, but the announcements of the events will always bring hype, discussion, and promotion to the game.
Not a lot of people know that if you change the date of your computer, you get a couple of Halloween-themed events... MOST of the people don't even know there are Halloween events. And I'm really hyped to see the reactions of new players to enter the game and, without updating or anything, have a couple of themed events after an announcement. I believe that the "surprise" effect the new users will experience will be beneficial for the game, and for them