First, the leveling experience is an area ripe for modification. It's something that everyone goes through, and as the game ages, is essential to modify to bring it up to modern standards. In some regards WoW has already made some pretty big changes here -- Cataclysm can basically be seen as a leveling revamp, and the instant 90 characters are another major leveling change (in that you get to skip the vast majority of it). However the base experience remains the same -- you complete a quest, kill something, pick something, and level. Changing this system isn't easy or foolproof, however it's an area that can be tweaked and iterated over with the right design. For instance I could easily see a leveling system based on lore and achievements integrate easily into WoW's existing structure.
The second area that WoW could push itself into new horizons is with the end-game synergy. Right now you have to focus on each end-game aspect and push into it by itself (PvE and PvP), and this is a good thing. While most people engage in one heavily and the other a little bit, there are extreme ends of the spectrum where players just do either PvE or PvP, and not the other. Integrating the two experiences, and creating synergy in the progression paths to allow for greater cross-over, can become a forefront that WoW could push into. It could be as simple as PvP gear dropping from Garrosh, or PvE rewards coming off of challenges completed in Battlegrounds.
Finally, as WoW undergoes its most significant graphics upgrade since release in the new character models, the game is open for progressing its base content consumption platform in a significant way. It's not possible to play WoW on consoles given the heavy use of the keyboard and mouse; however with those two devices still present, it would be entirely possible for WoW to become the first major RPG to use VR goggles, like the Occulus Rift. The world environment is already there, and it would "just" take some good client redesign work and UI modifications to make everything happen. I don't think changing the basic way we consume WoW of the question -- Blizzard could push into this space and be a pioneer.
With these three areas WoW can move itself and the genre forward. There are many ways that a ten year old game can still innovate and exist on the forefront of gaming and technology. Blizzard has the creative and technical talent to pull these things off. It will be interesting to see how their long-term roadmap takes shape in the development of WoW. But in the mean time, as we wait for these new things, its fair and necessary to look at just what WoW can do to push itself to the next level.
*I want to drop a note that this isn't a critique of the features of Warlords of Draenor. I think the expansion has plenty to offer and is going to be a lot of fun for all the different player segments of WoW.