I'm not aware of any buffs with this particular design already in game, but I've seen it used in other MMOs. It's too late for this expansion so perhaps a future consideration.

Something like:

Pre-order/early access - Grants 40 hours (arbitrary/debatable) of early access prior to official launch.

With the buff diminishing only while online.

This would encourage people to log out when they are done playing, not waste time/server space idling about, and hopefully allow more players through as others reach their time limit.

The buff itself would just allow access to new quests/duty registration. Once the buff expires quests/duties would again be locked until official release.

IDK, just something that came to mind. How difficult it would be to implement vs. how much it would alleviate congestion.. Feel free to discuss.