This was a feature on an old MUD I played many years ago.
What if attackers gained 'blood' whenever they attacked another player. This 'blood' would increase in time depending on the disparity of levels between the attacker and defender.
Anyone with 'blood' would then be open for attack anywhere.
In 2ho, where we all zone in and out of whompas/bunkers and tab and debuff, we'd gain this 'blood'. Sure we could zone to run away, but they can zone and chase you.
Concealment would be used more often. As well as perception. Fights could expand beyond arena/mmd/2ho.
Would make things a little more fun, and perhaps open up the outdoors a bit. :P
Maybe throw in another map upgrade that would show people with 'blood' as black/blue dots or something as well.