So wow vanilla had so me events during its time period. There are: the elemental invasion, Scepter of the Shifting sands, war effort and scourge invasion. Some were one time event and some were constant. Players who will play from the beginning of the server will experience them all. But as the content continues and new players come later they wont experience it at all. And my suggestion is make those events rarely, like the elemental invasion and the scourge invasion. This will make the server fresh and people want to kill those mobs all the time, I even think there are some quests where you can get some decent blue gear. Scourge invasion is introduced in patch 1.11 so this would happen when naxx is launched. But the AQ events (Scepter of the Shifting sands, war effort) will only be one time event, you cant open the gate once in a while so this wont be a good idea. But those two I hope you will consider them as maybe onece a month event.
P.S. Sorry for my bad english.