As it is, the playerbase currently has very little incentive to leave a capital city. Battlegrounds? You get teleported as soon as it starts. The same with Arena and Dungeon Finder. Raid? You can travel to the raid instance for a minute or two, or just slack and have your raid summon you.
I mean, sure, there are few situations. Like, if you're hunting for achievements. Or going to old school raids for some mounts or whatever. Or you're going to farm something. But those are finite circumstances and don't help that much.
What's worse, even the community itself feels dead. Oh, sure, if you're a raider you have your guild to talk to. If you're in an arena team you probably have a few friends playing arena. But outside of that, social contact in WoW seems pretty much dead.
I personally blame Dungeon Finder, cross-server battlegrounds (and even guild leveling to an extent) for this, but I cannot realistically expect Blizzard to get rid of those features any time soon.
Basically, I wish Blizzard would implement features that would encourage players to bond with other people belonging to the same server to foster a feeling of community. To make the world more alive by encouraging players to travel around Azeroth for whatever reasons.
I admit, I personally can't think of too many ways for Blizzard to do this, but I do have some suggestions.
For example, reviving world PvP. Yes, there is Tol Barad, but that's just one zone, and it's hardly world PvP when it happens in just one place. So, here's an idea. Make a daily quest. It would require you to kill X opposite faction players in Y zone. Also, the quest reward should be something significant enough for people to bother with it, but not too significant. Maybe make the quest reward as many Conquest Points as a single arena victory? Every day, the quest would take place in a different zone. And obviously, only players that award honor points would count.
Another idea would be to encourage people to raid other faction capital cities. Maybe make a quest with a very decent reward to kill an opposite faction faction leader? Making this a daily or a weekly quest obviously wouldn't work, but what about making it a monthly quest? Make it so the faction leader in question changes every month, and once you kill him, you get the full weekly amount of Conquest Points and some gold. And while we're at it, give a buff to the faction leaders. "Honorable presence" would be a nice name. Make it so whenever you're within X radius of the faction leader, you get 10x honor. This would encourage people to defend their faction leader and fight within the factor leader's presence (Buff would work for both factions.).
But this shouldn't be a PvP thing only. PvE-orientated players should also be encouraged to group.
Back in vanilla, we had Tyr's Hand and similar places where people would group to take out tougher foes and farm the rewards there. But I somehow think that wouldn't work in modern WoW since very few people, to my knowledge, actually go around farming gold. Still, trying to re-implement group farming into the game would be a great addition. I personally still have fond memories of farming Tyr with friends, trying to avoid the other faction and getting the hell away from that bloody patrol.
Another idea is to implement outdoor bosses. I understand how in vanilla they had that, but decided to get rid of it because not everyone could get the kill. So I propose a slightly different way of handling those outdoor bosses. Bosses themselves should be spread out across Azeroth and have a relatively short respawn (Ten minutes at most) time. But they themselves would drop no loot. Instead, there would be a weekly quest which would require you to kill one of those bosses. The weekly quest should rotate every day (So you get a different boss every day, but you can only do it once a week.) not to have everyone farming the same boss. The quest would only be doable in a group and the outdoor boss should be slightly stronger than a heroic boss. The quest reward should be a decent amount of Valor Points (At least as much as you get for completing the first heroic, preferably even more.) and a bag of goodies containing some gold, flasks, random heroic-level blue and a lower chance for a BoE epic. Maybe, alternatively, make it a 10 people raid boss, though that brings up technical difficulties about how to limit the group fighting the boss to 10 people?
I would personally love it if Blizzard implemented some incentives for people to group up with other players from the same server before signing up on Dungeon Finder, but that would obviously conflict with their current design philosophy. How about maybe making it so when you use Dungeon Finder, the system itself looks to mix you with players from the same server as much as possible? By grouping with players from the same server, using Dungeon Finder would be less boring as people would be less "anonymous" and you'd be more likely to communicate with each other as you'd have a higher chance of running into one another.
Of course, it would require a lot more than just these few ideas of mine to be implemented to "revive" the world. Obviously other features should be implemented as well, though I personally can't think of many and would like to encourage other people to post their own ideas about this in this thread.