Please, is this possible?
At training camp a npc asks you if you wanna join one of two factions, then teleports you there (i.e. Yuno or Prontera). After you've made your choice, pvp is always enabled against players who chose the opposite faction, even on fields and towns themselves. (If possible, Regular PvP areas lets anyone kill anyone as if this script wasn't in place.)
Basically a "vanilla wow"-like forced open world pvp setup!