I just wanted to add my 2 cents here....
The beauty of speculation is that no matter what you say, even if it is the wildest and craziest idea ever (like...let's say Thrall killing Ysera...) you have a chance to be right until all this info is confirmed/denied at the Blizzcon...
Regarding the race/class changes, there is a lot of reasons you could deny them or accept them based on lore..I would say this happens cause WoW's lore is not complete yet and Blizzard could open any door or window they want. If this was LotR and you come and say that they will get orcs fighting against Sauron, all hobbits going to Valinor...we could laugh our asses off that cause that story is complete. In the other hand WoW is not like that, is a "living world", we "live" the present of it and things could change in any possible way...Taurens may look for the Light as they see how pure it is dealing with the scourge..I would like to know what dk taurens tell Cairne about Tirion's use of Light and how it threatens the Lich King himself to the point he rathered leave the battle at Hope's Chapel (well the ashbringer did its part too...).
Before WC3 you could never ever thought of Horde and Alliance fighting together, Medivh being a savior but Blizzard wrote more lore and things changed, just like they might change now. If we keep living under the same "lore restrictions" we know so far the game will just either die or get freaking boring (at least imho...).
We can see all those leaks cause Blizzard for sure let them go out and read the fanbase thoughts about those possible changes without having to reply and discuss about it...they can just take the feedback and know how much info they could release at the blizzcon without annoying players too much. I always thought Emerald Dream would be the next expansion not including too much about the maelstrom thing but in a marketing point of view revamping azeroth now could improve the graphics and keep people attached to wow instead of looking for better looking games.
Now, if Orgrimar happens to be destroyed I dont think Blizzard will just let everybody die down there, as Myrok pointed in other post patch 4.0 will come before the expansion. Wouldn't be awesome to have daily quests in Orgrimmar about evacuating the city? I don't play Horde, but the simple fact of doing something like that for your people and city really makes me think of playing Horde now just to see that...
You may say: "how the hell they will know they need to evacuate?"...As I said at the begining of this post, thats the beauty of speculation...so I am going to:
Some Deathwing's goblins manage to escape, they look for help with their own kind and they say they are neutral and they can't pick a side cause that would affect their business, so they decide to ask both factions, as usual, humans couldnt care less for other races (specially for different looking ones) and Varyan fires at them saying he has enough problems to deal with others'. They go and look for Thrall who - as usual - decides to help them. Goblins join the Horde, Thrall listens to the info the Goblins know about an incoming cataclysm, he is wise and prepares for it evacuating Orgrimmar (sorry i just love the idea of those kind of dailies...).
Garrosh is not left in charge of the Horde, he is just left in charge of the orcs while Thrall is away then he disagrees with the pacific way Cairne looks at things and the influence he has on a side of the Horde so he plans or allows Cairne to die. He could also set it up to look like Alliance did and have the war he loves and look for.
For the alliance part idk...i have always doubted of Fandral and I agree with Myrok too that he is either a Neltharion's agent now or Nelth himself. I know Fandral was never the same after his son's death in AQ and he could have been easily corrupted. The counterpart for the goblins escaping could be some highborne going to darnassus and asking for forgiveness and telling NE about what is going to happen cause they found it from azshara herself....(i really dont know how possible this could be, but dreaming is free so why not?) then since Malfurion is back he forgives them and they start training what is needed to face the cataclysm and guess what? they start training NE mages. Darnassus might have its own evacuation thing too../shrug
Why do I think Malfurion will accept them? simple, Blizz is trying to the balance the leaders and we have Garrosh being a dick like Varyan and Malfurion being wise like Thrall. Call it coincidence or whatever but the upcoming book is about Malfurion and it may give us a different impression on how he could react to the highborne...
Anyway this was just my speculation, u could laugh at it, hate it, like it, whatever, i just wanted to have a chance of doing it before we know what part of all this is info is wrong.
Also this post might have tons of lore errors that i really dont know and i apologize for that, by no means I am trying to say this is accurate, this is just my opinion of what would be cool to see....
PS: Myrok u rock!