Patch 5.4 has several new features I find interesting, namely virtual realms, flexible raiding and proving grounds. What interests me most about these features, and features from previous patches and expansions like scenarios, LFR, and challenge modes are what they suggest for other possible features utilizing the same or similar technology. We saw this with the introduction of the dungeon finder for five man groups - that technology was applied to holiday bosses, then expanded for LFR, which is itself being altered and expanded for use in flex raiding.
This leads me to contemplate ways to expand this, and give us even more new features making use of elements we've seen before. It's all speculation, of course, but we all do it from time to time. What do you want to see for the future of World of Warcraft?
Scaling older content
Using a variation on LFR's queueing and challenge modes ability to cap gear item levels you could easily make older content not only scale for player levels and gear, you could make it randomly queueable. Plus, with flex raiding, you could make these scaled older raids runnable with any number of people from 10 to 25, or perhaps even up to the 40 person cap that original raids like Molten Core, Blackwing Lair and AQ had.
Another possibility would be to use this to create 'raids' of older dungeons. Perhaps even returning the original versions in some cases - it'd be fun to get a chance to run old Wailing Caverns, Scarlet Monastery, Scholomance, Stratholme again with all the sprawl and multiple wings, and the return of the gear lost to time for transmog purposes. You could even do irregular sized groups, using flex raiding's scaling to allow players to run, as an example, all of Dire Maul with seven players if you happened to have seven friends who all wanted to go.
An end to fixed instance sizes
In fact, why not take that a step further? Why not let people run current dungeons with a flexible size of group? With scenarios already filling the role of 'easy point acquisition' we could retool future dungeons to allow from 4 to 9 players, as long as the tank/healer roles were filled. You wouldn't use this for the random queue, this would purely be for a group of friends who want to run some dungeons. I've lost track of times I've seen someone ask in guild "Hey, anyone want to run some heroics" and quickly either fill up, leaving other folks waiting around for another group, or ended up one person short. The dungeon finder is a solution to some of this, but why not put tools in player hands for forming their own groups based around the number of friends they have instead?
To some degree this isn't really feasible for PvP, because you have to make an effort to match group sizes and levels to make BG's and arenas fun. You could have a 'handicapping' system that matches, say, 10 high MMR players against 15 lower rated players, but how much you'd actually get out of that is debatable and we've already seen PvP move more towards a gear iLevel cap for more fairness, not less.
Giving more players more options
One tool I really wish World of Warcraft would steal from the defunct City of Heroes and the brand new Neverwinter is the idea of some form of player content creation. I realize there are pitfalls to any such feature, and it would take an extraordinary effort on the part of the developers to make sure that these pitfalls were addressed, to make sure you didn't end up with a glut of terribly designed dungeon crawls for max XP and loot. (One way around the loot problem is to simply have all loot from player designed content be randomized and out of player control.) Letting players have some kind of means to design content would not only be an interesting experience in terms of getting players to see how difficult it can be to design compelling content, but it would also help address issues like the seeming dearth of five man dungeons in Mists. If players really want a lot of five mans, let them make them.
There are probably tons of possibilities I'm forgetting, which is why you now get to run wild in the comments. What would you love to see? Please don't say player housing. Oh, all right, say it if you must, but you know so's every fourth commenter.