So, I have been thinking a lot recently with the recurring PvP discussions going on. I "think" but am not sure if I have come up with something that is a workable idea.
To be able to "make war" on someone else, you must first make yourself vulnerable "to war." As in, to be able to kill someone on someone else's Tribal Lands, you must first make yourself vulnerable to be killed.
This could also work the same way for a "Tribal War" system. As in, if you want to try to take over someone else's "Tribal Land" you must first make your land vulnerable to being taken over.
Example (only an example, not saying this would ever happen, lol): Regulators are sick of Hopi's, Hopi's are sick of Regulators. They both decide to attempt a war, make themselves vulnerable, and fight it out. Now, while they are fighting, the Templars decide they will try an attack while those two are busy, make themselves vulnerable, and go attack Regulators. Then, because Templars are hated by the DHoV guys, they make themselves vulnerable and attempt to take over Templar Land while they are off fighting.
Throw a 6 hour or 24 timer on changing the "vulnerable/invulnerable" option, and there ya' go. Sure, someone will have to be "first" to go vulnerable for someone else to try to take them over, but if months go on and no one does it, well, we're all bigger "carebears" then we claim to be then aren't we?