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.
People can't do any rules for PvP and enjoy it because the pvp itself is not balanced at all. The PVP in D3 is whoever is faster is the winner.
What we've seen in the trailer was that barbarian chasing the soso in D4. Was looking like a PVP in an open world. I would like and I believe will be arenas for 1v1 up to 3v3. And something simple so you can invite someone to duel wherever you are on the map.
I'm hoping they'll balance the heroes and the game well this time. Because to be honest first Diablo was released in '96 so they've got enough experience to sort things out.