I'm not sure how viable it would be... but if a player[s] had their base set as a safe zone, no pvp, would it be possible to set up a viable system where a tribe can flag another tribe?
So for an example.. tribe A has had enough with evil tribe [tribe B] in the area and kills a few of their players as revenge... after reaching [5 kills in a day? 10 kills in a week?] tribe A is now fully attackable for tribe B, as if their base was no longer a safe zone, for a set amount of time[2-4 hours?] this would give both sides the time to set up and organise attack and defence.
Even typing this I can see areas where this idea is weak... but is an idea nontheless