So, I'm not really that invested in PvP, I generally do not enjoy it and could take it or leave it, but... I had an idea and I figured I'd post it.
The thing that has always driven me away from PvP is getting ganged up on. It would just happen to me constantly when I did PvP... I couldn't tell if I was being targeted specifically, every time, or if it was simply a matter of the opposing team being organized such that they focus fired each of their opponents in turn, all five on one, and then moved on in cycle to another opponent while that one was sent to respawn limbo.
What if there was an incentive that encouraged PvP opponents to target people who target them? Here's my thought: if you fire weapons in a PvP and you aren't targeting another player who has you as their primary target, you get a short duration debuff (damage resistance, maybe). This would discourage the whole getting constantly ganged up on phenomena that drives me (and I would guess, quite a number of others) away. Furthermore, it creates a situation where you have a vested interest in matching up with one other specific opponent, mutually, so that you both avoid the debuff. The buff wouldn't stack and would go away very quickly if you stopped firing on PvPers who weren't targeting you. It wouldn't factor into PvE combat at all, whether in the main game or with respect to NPCs in PvP maps.
Good idea? Bad idea? Doesn't address any actual problem? I'm not sure. I will say that my intuition tells me there's a major flaw with this idea, but I can't think of one.