This goes along with the portal debate, but here's something I'd like to implement: a "beacon" that can be set on/off by someone in your party that allows you to port to their location. This could be a method of combating port-pking--if someone knows that if they port-pk, their whole party will come and kick bum, they might be less inclined to do so (or it might make it more risky!)
Also, on a personal note--I liked the availability of port-pking (even though I got port pked and lost a lot of items that way...) because it was a way to get back at higher level characters with little risk to yourself.