Take crom, as an example, and make one random zone pvp enabled each week. That means only one non-siege zone is pvp enabled the whole month.
Pvers would have 95% of the current server as pve, and pvpers wouldh have an active pvp zone. Maximun pop 24/7.
Many ways to go around pvp zone:
1. Go to another zone
2. Go to another instance of the zone
3. Get friends or guild to participate in open pvp zone
4. Play another char or even wait until the pvp is a different zone, max of a week
Pvers lost one zone, out of the many, a week, but still have 95% of the game as pvp free.
This is the best way to put everyone together and maximize gameplay of all types.
Pvpers lose all zones being pvp, but still have an active pvp zone.
Max pop and activity 24/7, evrybody wins.