Bringing in PKing like in D2 wont work in this day and age. Back in D2 people just played games for fun, so they didn't mind just making PK games and spending huge amounts of time to build a dueling character even though there was absolutely nothing to get from it.
Nowa days unless there is some sort of reward, ranking, balance etc. to it almost nobody will do it. D3 is the perfect example of this, nobody PvPs in D3. The option is there, but there's nothing to be gained from it so nobody does it. People could make rules in it enjoy it just like they did in D2 but they don't.
Personally I'd like to see a runescape Wilderness like thing built, where the enemies had increased rewards but the zones were shared and pvp was enabled meaning players actually fought over the area to get the rewards. Something like this would marry the pve-loot farming thing of ARPGs with PvP. Although I fear this would end up just getting overrun with guilds of 20+ players effectively owning the zone for themselves so don't know how you would balance that.