A community gathering incentive.
Whether it's (just random ideas with no forethought):
the need for an "EC tunnel" like auctioning system
Dance parties
A small buff people recieve when they're hanging out around a magical area for a long period of time.
Instrumental or skill "jam" sessions that the more people who gather around and participate, people recieve stronger skill gains.
Sermons of some kinds that allow faction increases
Dance parties
Something to promote community gathering.
The occassional "event."
I remember in the earlier days of EQ when the undead rose up in the Qeynos Hills. It was pretty darn impressive having every single level contributing and taking out the skeletons. The zone was absolutely infested! High level players taking out the high level skeletons and skeleton casters as the lower levels helped out and ran for cover near the guards. There were deaths but oh was it fun. And as word got out about what was happening and more and more people showed up the zone crashed. But it sure was fun while those of us got to partake. This was on the Bertoxxulous Server maybe 2 or 3 months after release. I don't know if it occurred on other servers or not.
It was the only one I got to experience although I heard of the Crushbone Orcs trying to take over Kelethin.
Those kinds of things can absolutely make for a one month subscription right there. Unannounced and Big!