Quite simple, as others MMO’s like OSRS, you could add something like Wilderness, where you find more resources abundance, some instances/dungeons with epic loot or even new bosses.
It could be Lvl 60+ or even with gearscore cap to enter the zone (trough a portal, some npc to talk on a boat that or smth like that) to ensure some sort of “fair play” between players.
The thing is that players will fight each other for bosses/resources while they will get a higher death penalty for dieing, it could be exp/random drop from equipment.
Thats it, quite simple and it will add a different dynamic to the PVP, getting something different from that PvP enable/disable thing