I REALLY doubt we will see a new class next expansion, Blizzard has repeatedly stated their desire to be very cautious when doing that. I think the better question is what is the new main profession we will get?
I would love to see two new proffs:
Woodsman: Chop down certain types of trees for their wood (gathering)
Fletcher: Makes Bows, crossbows, staves, and polearms from woodsman mats (crafting)
Wait why do people associate Turaloyn/Alleria with the Legion. If my lore is correct they went into the portal chasing the horde not the legion.
The horde was largely a faction within the legion at the time when they followed them through. The general consensus (given Blizzard saying we'll see them soon TM) is that with the return of the legion, whatever portal they went through will be reopened. We'll likely see the results of where that group of horde went as well as what those two have been doing to stop them.
On a related note, it'll be cool to see one or more of the following in relation to their 10th anniversary.
* 10 levels (plus pet levels and guild levels)
* 10 new race models (all vanilla + draenei and blood elves)
* 10 heroic dungeons (some favorites revamped for the anniversary and the new storyline, some brand new)
* 10 boss entry raid
* 10 paths to follow in a revamped Path of the Titans (want to tank, take this path of the titans, etc).
* 10 PVP features (mixed up between BG, arena, world pvp)
* Release date: 10/10/2014 or 11/25/2014(closest Tuesday to 11/23, the original release date).
Whenever the Burning Legion expac comes, there's really no other credible choice for a new Hero class, or standard class, than Demon Hunter. Blah blah to all mention of them being in Diablo 3; different game, different mythology, so who cares? It is a hugely iconic lore class that directly relates to the proposed content. Doing a major Burning Legion expansion without Demon Hunters would be like if they had done Wrath without Death Knights.
I was going to type exactly this when I saw that you had beaten me to the punch :P
If the xpac is Emerald Dream, I want Ogres and Naga or (preferred) playable Dryads/Nymphs. A playable Mylune ftw :P
Given the fact Blizz has given us no lore hooks to the emerald dream, and that the major plot there was wrapped up already in Stormrage, I find it doubtable we'll see that expansion. But a Great Sea expansion could still fit your idea, sans Dryads.
Niellak, you seem to be very mistaken on what a Demon Hunter actually is in the context of Warcraft lore.
They are a primarily melee class who uses demonic magic to enhance their fighting abilities. While they are not completely devoid of ranged abilities, giving them even a single dedicated ranged spec would be odd. Likewise they are portrayed universally as being unarmored, and while this is obviously not an option in WoW, anything beyond leather armor would be stretching their theme past the breaking point.
Overall, while they might fit in very well in a Burning Legion expansion pack, I honestly don't see them as being a viable class in the forseeable future. In a mechanical sense they don't really fill any niches that need filling. From a lore perspective there is very little they can do that Warlocks can't, plus their presence would screw Warlocks out of an expansion that could of been their time to shine.
Would be nice if they use the next expansion to expand on the concept of Warlock Tanking and make Demon-Form more akin to Guardian Form. Ahhh dreams...