Moment in time
First, we need to clarify when all of this is happening. Garrosh Hellscream is traveling back in time to some point before the orc chieftains drank the Blood of Mannoroth -- in plenty of time to stop the ceremony and offer the chieftains an alternative that doesn't involve allying with the Burning Legion. Because of this, there are several moments in time that simply cease to be on Draenor -- and people that we won't be seeing as a result. In this version of Draenor, these people simply don't have a way to access the world, or even exist as far as we know.
In the original timeline, the Dark Portal was not constructed until well after the orcs drank the Blood of Mannoroth, which means that on this Draenor, it isn't actually there -- or if it is, it has taken on an entirely different purpose. Our version of reality is being connected to this alternate version of Draenor via our Dark Portal, so it only stands to reason there is something on the other side. Whether that is an alternate version of Draenor's Dark Portal or not remains to be seen, but we can be fairly certain that whatever it is, it is not the Dark Portal we're accustomed to in our reality. It might look like it, but the reasons for its construction and the reasons for its existence are entirely different.
In our reality, the Alliance, Deathwing, Illidan, Kael'thas, Vashj -- all of these people traveled through the Dark Portal, but it doesn't exist on this version of Draenor. History has changed, and there was never a connection between that alternate version of Draenor, and its correlating alternate version of Azeroth. There is no way for these people to get to Draenor, not to mention even know about Draenor's existence in the first place -- so we aren't going to see them. If there is an alternate version of Azeroth floating around out there, we have no way to access it or see it, and it has no way to access or see us.
Obviously there are orc heroes on Azeroth that well remember the events before the invasion of Azeroth, when Draenor was still in one piece. However, it's been stated that we will not be seeing any duplicates of existing characters on this version of Draenor. We won't see two versions of Velen standing side by side, Saurfang will not be running into himself, Drek'Thar will not have an opportunity to go back and visit a much younger, more spry version of himself. This has been stated, but we haven't really been given a reason why this won't be happening.
At this point, all we can do is speculate about it. And really, the best reason I can conjure up for this circumstance is that in the reality of the Warcraft
universe, there can only be one version of a person in existence at a time. Fanciful tales of time-travel like those found in Doctor Who
deal with multiple versions of people and the nature of paradox -- but Warcraft
isn't that kind of story. If Drek'Thar, for whatever reason, travels through that Dark Portal, he will continue to be the only Drek'Thar, and his alternate version will simply not exist on Draenor. Or if he does, he's nowhere to be found.
Alternatively, our present-day versions of these heroes are simply not going to go through that Dark Portal at all. Velen was listed as one of the heroes of this expansion, but we don't know if that's the Velen of our reality, or the Velen of this alternate Draenor. The description suggests that it's our Velen, from our Azeroth -- but we'll have to wait and see to determine how that plays out. As much as we might like the idea of alternate versions and duplicates of characters, as fascinating as that might seem to be, the developers have stated that this isn't something we're going to see happen.
Because this is an alternate version of our timeline, there are things that exist in present-day Outland that we aren't going to see, either. Tempest Keep and the majority of the naaru in Outland, including A'dal, did not actually arrive on Draenor until after its destruction. They didn't make an appearance on the world until after it had been shattered, well beyond the point in time that the orc chieftains drank the Blood of Mannoroth. Because of this, we likely aren't going to see A'dal or the Sha'tar make an appearance -- and we won't see Tempest Keep. In this version of reality, it never came to Draenor.
This doesn't mean we won't be seeing any naaru at all. Oshu'gun, the great diamond that juts from Nagrand, should still be there. Oshu'gun was the dimensional ship that the draenei used to flee Argus. It's the ship they used to get to Draenor, and the naaru K'ure was the first naaru to make contact with Velen all the way back on Argus, before the draenei fled. In the original version of Outland, K'ure fell ill shortly after Oshu'gun landed, and began turning into that darkened state in which it is first encountered in Outland. If we do happen to see K'ure, it's likely that we'll see it in the same state as when we found it in Outland -- darkened, or on its way to being darkened, and not a lot of use to our purposes.
Similarly, the naaru D'ore "died" when Oshu'gun arrived on Draenor. In its darkened state, D'ore was the first naaru to be interred in Auchindoun. We'll be seeing Auchindoun -- it will actually be a dungeon in Warlords
-- so we might encounter D'ore. But if we do, it's going to be that darkened version of D'ore. Any other naaru we presently know about was a member of the Sha'tar, or allied with the Sha'tar, and will not be found on Draenor.
Magetheridon and the Burning Legion
What about Magtheridon and the Burning Legion? The history of the Burning Legion on Draenor is well-known -- after all, it was the corruption of the Burning Legion that caused all our troubles on Azeroth in the first place. In this version of Draenor, the Burning Legion has not simply ceased to be. Gul'dan is still shown as having green skin, even though the other orc warlords have obviously not traveled down the road of fel corruption. We don't know if we'll see Kil'jaeden, Mannoroth, or even Magtheridon in this version of Draenor -- but the evidence we have so far suggests it's a very likely possibility.
So here's what happened, in our original timeline. Kil'jaeden discovered Draenor, and also discovered the draenei -- and he wanted vengeance on Velen and those that had fled Argus so long ago, turning down Sargeras' offer. He was also supposed to be finding a mortal army that could be used to conquer worlds, and Draenor offered not only the orcs, strong warriors capable of destruction, but a chance to get back at Velen. Kil'jaeden went to Ner'zhul in disguise, pretending to be the spirit of his dead mate, Rulkan, and convinced Ner'zhul that the draenei were plotting against the orcish race. This started the wars between the draenei and the orcs -- but the ancestors were no longer on speaking terms with Ner'zhul, and that caused him to eventually discover the truth at Oshu'gun.
It was at this point that Ner'zhul's powers were stripped from him by Kil'jaeden, and Gul'dan was given the reins. And it was sometime after that that the orc chieftains were given the opportunity to drink the Blood of Mannoroth, and sometime after that that Gul'dan was contacted by Medivh, and constructed that first Dark Portal. Without knowing the precise year that Garrosh arrived back on Draenor, we can safely say it was at some point while the orcs and draenei were at war -- but before Gul'dan's treachery, before Ner'zhul discovered the deception, and before the Blood of Mannoroth was ever offered. So the draenei and orcs are fighting, yes -- but the Legion's influence over the orcs has been shattered, for the most part. Medivh isn't even a player in this game, from what we know.
Although this is a brief list of what we likely will and won't see on Draenor, keep in mind that this list is not set in stone. Obviously the Creative Development team at Blizzard can change history as they see fit, or bring in characters we weren't expecting to see. Who knows, A'dal and his team of naaru might show up at some point just to see what's gone wrong on Draenor -- or some strange portal might actually link up the Draenor of this reality with Azeroth. It isn't likely, but it could happen -- we just don't know enough at this time to really speculate about these things, so it's best to simply go by what we've been told so far.
For now, here's what we do know: We'll be encountering those orc chieftains of old. They've been somehow duped by Garrosh Hellscream, who has apparently united them all into the Iron Horde and intends to attack our version of Azeroth with this legion of orcish might. But beyond that, the skirmishes and fighting between orcs and draenei over on Draenor have yet to cease. On Draenor, the draenei race still has not been wiped out like we've seen in our timeline. Shattrath stands tall, the Temple of Karabor is still very much a draenei structure, and Auchindoun has yet to fall to ruin.
We don't really need to concern ourselves with alternate timelines and alternate realities. The danger being presented here is very clear, and it's something that needs to be stopped. Once we've finished putting a halt to the threat of the Iron Horde, once we've saved the draenei from destruction at the hands of the orcs, that's when we can step back and assess exactly what's going on and how it changes what we know about timelines and reality. But that moment is a long way off in coming. Until then, we should sit back and enjoy the ride -- because it has been stated that the repercussions from this journey into this alternate world will be felt for some time to come, which means we're in for one hell of a story, even if we aren't exactly certain what's in store.