This one's simple enough - take what people like about random instances, combine it with arena-style PvP, and see what happens. Imagine if you could randomly queue (like a modern BG) for a group with up to five other players, and be randomly ported to one of several spawn points inside one of the many instances World of Warcraft has seen over the years. So as an example, if you queued for this feature, you and four other players (either a premade group of friends or a randomly matched group) would be ported to Stratholme, in one of several randomized spawn points throughout the zone, and so would an opposing force of players seeking to destroy you.
Some dungeons would be more suited to certain kinds of play than others - dungeons which are basically linear would see groups trying to arrange choke point ambushes, for instance - but the fact that the map would be randomly selected and could be any out of the vast array of dungeons currently in the game could make for interesting play options for PvPers, and give people who don't currently do a lot more exposure. And because it's a dungeon, you could use the same scaling as Challenge Modes and make sure all players are on an even keel, either as default or as a selected mode toggle for added bragging rights or even special rewards. I can imagine a cosmetic set of PvP gear available through these 'Challenge Mode Arena Dungeons' where everyone's gear is normalized and skill is the great variable.
This is one I've half-thought we'd see for a while now. With the introduction of mobs that scale up when multiple players tag them and the ability to artificially raise or lower player gear (seen in Challenge Modes and PTR testing) we inch closer and closer to the ability to scale the bosses in older raid and dungeon content to be a challenge to the modern player. As much fun as it is to blow through MC taking out raid bosses in two hits, it might also be fun to actually have to pay attention and work together with a group to take out Lucifron. (Well, okay, he was never all that hard.) Especially as we move forward in time through the raids, some of the fights have some interesting mechanics and it would be fun to get to experience them scaled up to still matter, like Teron Gorefiend or Mimiron.
Scaling of this sort could also make it so that queuing for a random heroic wouldn't just have the current crop of dungeons, but every dungeon.
Non-instanced World Content
To be honest, we already see a good amount of this with daily hubs and world bosses, but it used to be the case that areas existed in the world that were effectively dungeon complexes, but without the instancing, and while I don't think they should be the rule I wouldn't mind seeing more of that kind of thing without it being tied to daily quests. Imagine a crumbling set of ruins with several elite mobs prowling it, each dropping dungeon quality loot on a relatively short respawn timer, but with limit on how often you can get loot off of them. Consider them mini-world bosses if you like, similar to figures like Garr in Cataclysm. By tying them to an area, making them spawn more often and then making it so that camping them all day wouldn't get you any more reward than just killing them once and moving on, you encourage people to put a group together and go clear out the area. Think a beefed up Jintha'Alor.
The Arathi Offensive
Okay, this one isn't going to happen. Just imagine I talked about how cool it would be to lead a crack team of Alliance veterans to retake Stromgarde, or a burly band of Horde bullnecks to seize control of Refuge Pointe. I frankly doubt that the new feature would be a redesigned Arathi Highlands, but it's fun to dream.
So these are some of my ideas. I still suspect the new feature will be the Proving Grounds, but until it's announced, all we have are guesses.
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