Heernis
My idea on the matter is that you switch back to faction based PVP when you're labeled. So that means:
1) Kamis attack Karavan and Marauders, but can't be attacked by Rangers, except in a Karavan or Marauder's team.
2) Karavans attack Kamis and Marauders, but cannot attack Rangers, except on a team of Kamis or Marauders.
3) Rangers attack Marauders only, but cannot be attacked by Karavan or Kamis, except in team with Marauders.
4) Marauders attack Kamis, Karavan and Rangers, as well as all 3 can attack Marauders. (Addition: Marauders can attack each other)
Since it depends on fame, all of them under fame 25 or above -25 cannot be involved in any fight.
Your goal may be to promote PvP, but your proposal was to restrict whom is able to attack/heal who.