I'm going to go out on a limb and say that Expansion 5 is going to be a game changer. By all rights, this next expansion will be the expansion in which the 10th anniversary is celebrated, and while I think there's no doubt that it will be released prior to that anniversary (November 2014), I think the fact that it will be the '10th anniversary Expansion' will be played up, and that there will be a bit more 'oomph' for this expansion than there has been since WotLK (which, no matter how it was received, was probably the overall most anticipated expansion to date due to the popularity of Arthas/TLK)
So, I expect something big - comparable to the inclusion of the Lich King - to pop up in the next expansion. These bells and whistles won't just be for the anniversary, however - they'll also try inject new life into the game (which is far from dead, regardless of what anyone says, but it slowly diminishing in population). This considered, here are my (completely wild) guesses as to details on the next expansion:
Demon Hunters as a Playable Class - This is unlikely, and many would consider it ill-advised (coming so soon after another new class), but I think it's coming (If not in Expansion 5, then eventually). Demon Hunter is likely the single most desired 'hero' class and its inclusion alone would draw in many new players and many 'retired' vets. I can see Demon Hunter being implemented in two ways. First, as a Hero Class that starts at 85 or so, and is treated much like DKs were. Second, as a potential "Heroic Specialization" (read: 4th spec) for rogues. I won't go in to how I think 4th specs could be introduced, as that would take far too long, but suffice it to say I think it could be done quite well.
Illidan's Redemption - Haters gonna hate, but mark my word - this is coming. Blizz already practically confirmed Illidan's return at Blizzcon 2011 when it was said (direct quote) "It's likely we will bring Illidan back". Illidan is one of the most popular characters of all time, and his re-appearance (if treated correctly) would thrill many fans. It would also go hand-in-hand with the inclusion of the Demon Hunter class. I say Illidan's redemption, because I don't think we'll see him as an enemy. Even in Outland, he wasn't super villainous, and it can be argued that he had the best of intentions starting out. This is all the more likely considered the continued development we've seen of Illidan's character (Illidan appeared in both WotLK and Cataclysm, and while he doesn't directly appear in MoP yet, his motivations are made cleared through the new Warlock quest line.)
The Emerald Dream - Originally rumored to be an endgame patch for vanilla, The Emerald Dream has been the source of fan speculation for nearly a decade. Though much of the Nightmare story was dealt with in the novel "Stormrage", there is still a great deal unknown about the Emerald Dream. I think the Emerald Dream provides several opportunities for furthering the story in WoW at this point. First of all, I think it can help facilitate Illidan's 'redemption' - with the help of Malfurion, Illidan's spirit can become the guardian of the dream (replacing Ysera, who like all the former dragon aspects, has dwindled in power.).
The Emerald Dream also provides opportunities to further the story of The Old Gods and the Titans. The Dream is a blueprint of a pure Azeroth created by the Titans. I speculate that it is a 'failsafe', a means for the Titans to affect 're-origination'. Furthermore, I expect the Titans will actually be enemies for a brief period. With so many noteworthy things happening (Algalon, Uldum, Deathwing, Ra-Den), Azeroth is on the Pantheon's mind, and re-origination is already established as their 'solution'. This of course would be something that we, the players, don't want, which would pit us against them. And then there are the Old Gods - the corrupting influence that created the Nightmare. If the Dream is a blueprint for re-origination, it's likely the Old Gods' corruption would need to be dealt with first
The Caverns of Time - Always a fan favorite, I think the Caverns of Time will play a vital role in the next expansion. The Timewalkers are a faction introduced in the WoW card game. They are a group of mortals who have taken it upon themselves to preserve the work of the now diminished Brzone dragonflight - guarding the timeways. While Cataclysm brought a fairly anti-climactic 'end' to the Infinite Dragonflight's story, that timeline has now ceased to exist (as Deathwing was killed before it could ever occur). This means that the Infinite Dragonflight could still be out there - even considering Nozdorumu's diminished power (in fact, this could be key to his downfall). Through new CoT instances, more foreshadowing can be presented for future stories, allowing stories that aren't directly addressed in expansion 5 to remain active. [Personal Note - how cool would it be to do an instance where members of the Timewalkers come to the present to prevent the Infinite Dragonflight from meddling there? Kind of a role reversal]
But what about the Burning Legion? It's undeniable that Wrathion's quests appear to point towards the next conflict with Sargeras' Burning Legion, as well as Velen's prophesied Grand Army of the Light (led by Anduin). However I don't think it will immediately follow MoP for many reasons. First, the Alliance and Horde will need some time to simmer down after MoP. I doubt they'll be in a position to immediately join forces so soon. Second, there is still a lot of character development that can be done, especially with Anduin. Finally, I think they'll save a Burning Legion expansion for awhile, keep it in their pocket for when they really need to revitalize the game even more so than I'm suggesting here.
As far as mechanics go, I have one big expectation. All content up to WotLK will become free to play. F2P is the new normal for MMOs, and WoW will have to try and compete at some point. I think they'll test the waters by expanding the current 1-20 free to play into something much more broad.