One of the ideas mentioned at Dragoncon was creating an "Adventure Mode" for WoW. Adventure Mode is a feature in Diablo III that allows players to travel freely between the different Acts and the areas within, playing where they want to and completing "bounties" (essentially quick quests) to get useful rewards.
Doing so in WoW would have to be a pretty different affair. Since Reaper of Souls, Diablo III has had enemies in any area scale to your level, meaning that leveling up (at least until you get Paragon levels) is more about getting new abilities and runes (modifiers to abilities) than becoming numerically superior to your enemies. In fact, you could beat the entire game in its ordinary mode at a fairly low level, because enemies scale down as well as up.
Azeroth (and Outland/Draenor) is, of course, a much larger setting than Sanctuary, and WoW's gameplay is designed for more discrete chunks - specific quests and objectives, specific trash groups and bosses. And WoW is not automatically scaled to one's level (though Timewalkers have proven that it's a technology they can implement, at least in some situations.)
But Diablo is also randomized by its nature - not only are its many rare/elite enemies given random affixes (abilities) but the maps themselves are procedurally generated when you enter a dungeon, ensuring that you'll always have to do some exploration to get where you want to go.
Adventure Mode in WoW could take a couple of forms.
One would be to allow you to go back and do entire zones again, either scaled down (but like Timewalkers, retaining all your abilities,) or scaled up, and potentially then getting scaled-up rewards.
The other option is to create new randomized daily (or weekly, or whatever) quests that send you to a random zone from older expansions. Perhaps one day you'd get a quest sending you to Redridge Mountains to fight some bandits and another day you'd get a quest sending you to Sholazar Basin to fight some Cultists.
I think that of the two, I'd prefer the former. One of the things that has happened over the years is a kind of irreversible raising of stakes - at this point, the player character is such a big deal that everyone is practically kissing your boots as you walk by. You're the Commander, and soon you'll be the leader of your entire Class Order.
That's fine, and certainly makes sense given the things we've accomplished (killed the Lich King, two Old Gods, beaten both co-leaders of the Burning Legion - one of them twice,) but there's also something really appealing about just being the wandering hero, going to strange towns and fighting bandits. These days, you pretty much have to replace "bandits" with "highly trained military hit squad empowered by dark magic" to make the threat seem plausible.
Anyway, there hasn't been an official announcement of this feature, so I wouldn't get too hyped up about it just yet - it's the kind of thing that might be delayed or cut to ensure Legion comes out in a reasonable amount of time (and man, I'm starting to feel done with Draenor - sad that that's less than a year after getting there.)
Especially given the fact that we're now in the era of level boosts, and that Demon Hunters are going directly to the Broken Isles after finishing their starting quests, it'd be nice to have an incentive to take people back to older content.