If we're talking about "world" PvP, as something distinct from Battlegrounds, arenas, duels and especially Wintergrasp, I think the way to revive world PvP is to incentivize it *heavily* and do it in a way that brings the two factions into close proximity of each other.
For those of you who played at the end of TBC, the Isle of Quel'danas was frequently a great place for world PvP even on PvE servers. I recall anywhere from 5 to 40 flagged players hovering around those quest hubs at any given time. All it took was making a zone that concentrated players of both factions into a small area on a persistent basis.
Blizz has tried to boost world PvP half-heartedly in the past, recall the quests in Silithus that would flag you for PvP while you were doing them. Same thing in WotLK with the Grizzly Hills PvP quest hubs. But the trouble with those quests is that the incentive to do them other than just for the fun of it are really lame. You get a few gold and some useless tokens.
If there were a "contested zone" on all server types like Wintergrasp, but where you didn't have to queue for a "battle" and were flagged as soon as you entered that would be a start. Then they'd need to add significant incentives for doing daily quests in this zone. Substantial rewards, like mounts, titles, crafting patterns and other *vanity* items along with heaps o' gold would really get people interested. Rare vanity items similar to some of the rewards you get from the fishing daily quests would keep people interested over the long-haul. If there were anti-air cannons or some other way to get people off their flying mounts and into the fight it would also help keep this zone a hotbed of world PvP activity. But (FFS) don't put this zone in the center of the continent so people who want to avoid PvP aren't shot down every time they try to go anywhere like the original WG did.
If the daily quests involved competing for certain items or resources it would really heat things up on a persistent basis, unlike WG which is mostly quiet except during the specific battle times. You could even have quests in the zone that just asked you to kill "X" number of enemy players to guarantee there would be skirmishes throughout the zone.
I think this could be a fun way to expand on the current PvP options in WoW and keep people interested specifically in what's going on between factions on their own servers. BGs, Arenas and the Random Dungeon Finder have made it so many people play while barely interacting with the people on their own server. A contested PvP zone like this would focus players on their own servers again and bring world PvP back to the forefront.
Edit: TLDR - Create a PvP "zone" on all servers like WG with meaningful incentives from daily quests to foster world PvP action.