We're still probably over six months away from 7.2's Tomb of Sargeras patch, so I'll admit it's a bit early to speculate on this. Still, going to Argus is one of the huge "checklist items" in World of Warcraft - something that we have been wanting and expecting for ages.
A little history of the Draenei: in Warcraft III's Frozen Throne expansion, you actually spend one campaign (ironically the "Alliance" campaign) playing as Kael'thas and then Illidan. Interestingly, Kael'thas is pretty much unmistakably heroic and motivate by good reasons, making his eventual transformation into a totally evil villain in Burning Crusade pretty shocking.
Anyway, you travel to Outland to escape the Scourge and the Alliance forces who wanted to jail you for cooperating with the Naga, and it's there that you meet the Draenei, led by Akama.
The Draenei in that era were depicted as A: having Lost One models, which we now think of as basically Broken-squared and B: as, well, what we now think of as Broken. Akama's probably the most prominent Broken character (him or Nobundo,) and there's no indication of the healthy Draenei and where they came from.
Of course, in Burning Crusade we had a big retcon (actually, it was an accidental retcon apparently, as Chris Metzen had forgotten the lore he wrote - I bet they're more careful about that stuff these days.) The Draenei originally had nothing to do with the Eredar, and in fact, in the original lore, it was the Eredar who corrupted Sargeras. Post-BC, the canon (that we have to this day) is that the Eredar were originally a highly advanced race living on the planet Argus, most of whom were seduced by Sargeras' power and transformed into Demons, while a portion of them instead fled, becoming the "Exiled Ones," or Draenei.
The Draenei are profoundly long-lived - in fact, they are the longest-living playable race in the game. The Draenei left Argus either 25,000 years ago or 13,000 years ago (the latter value seems like the more recent retcon,) and many Draenei, including playable characters (definitely all Death Knights) were born on Argus.
Draenei have spent ages - literally ages, because we're talking about having left three thousand years before the Sundering on Azeroth - fleeing the Burning Legion. And that is about to end.
Leveling up in Legion, there's an event that I'd argue is sort of this expansion's Battle of Undercity (though thanks to scenarios, it should survive even if Azuremyst Isle winds up getting revamped.) You bring Light's Heart to the Exodar, only to find the ship-city under attack. You eventually discover that the leader of the assault is actually Velen's son - a son that was stolen from him by Kil'jaeden after Velen rejected the Legion's offer. After he is forced to watch as his own son destroy a Naaru and then get killed by us, Velen declares his new intention: they will have the Exodar repaired and they will return to Argus. (If you haven't seen this dialogue, when you do the scenario on a subsequent character, click the teleporter item Velen gives you at the end but then cancel the cast to see a little extra scene.)
For a Draenei, this is what you have been waiting thousands of years to do.
So, looking forward to 7.3, here's what I'm expecting and wondering about:
Everyone On Board the Exodar:
Given that we'll be presumably closing the portal at the Tomb of Sargeras when we do that raid, it's not like we're going to be using that portal to get to Argus. Thankfully, we have a dimensional ship that we're about to get in working order. I wonder, then, if the Exodar will serve as a sort of hub on Argus. Would it simply become a new neutral city? Or does the Horde have some alternative means of conveyance? Come to think of it, what about the Botanica, Arcatraz, and Mechanar? I mean, we cleared those out years ago, and they never crashed.
What Remains of Mac'Aree:
Argus was apparently a nearly utopian society before Sargeras came - the only thing he could offer the people there was knowledge. But it has been, you know, thirteen or twenty-five thousand years since the Legion took over there. Hell, a lot of the Legion's "look" may have been pioneered by the Man'ari Eredar (the demons sure use plenty of crystals.)
On the other hand, wouldn't it be ironic if, while the Legion spreads fel flame and corruption around the universe, Argus might be preserved.
Frankly, given that the Tomb of Sargeras is likely to be the most straight-forwardly "Felfire and Brimstone" looking raid, it would be pretty interesting to see if Argus looks nothing like what we'd expect.
In fact, given that Sargeras is a Titan, you could imagine that Argus looks less like the Broken Shore and more like Ulduar.
You Can Go Home Again, But Can You Stay?
Here's the really big question for the Draenei: They have spent all this time hoping that they can prevail against the Legion and finally reclaim their homeworld. But after so many thousands of years of demonic corruption, is that completely insane? Is Argus a world that can support life of the non-demonic variety? And is anything that the Draenei left behind still really there?
Obviously player characters are going to stay on Azeroth for the long run, but if there is an Argus left to salvage, wouldn't a lot of Draenei seek to return home and start the process of healing their world?
Our World or Theirs:
Ok, the next part has some spoilers from the Illidan novel (I haven't read it, but I've found out about these spoilers.) We know that demons can only be killed in the Twisting Nether - in a sense, that's where they always are, simply projecting themselves into physical reality. But apparently, if a world is so saturated in Fel energy, these worlds sort of count for that. And Illidan managed to travel to the Dreadlord homeworld, Xorroth (home of the Dreadsteed mounts that Warlocks get) and destroy it. Those Dreadlords who were on the world were, I believe, truly destroyed.
The Dreadlords have been a pretty key part to the Legion, and losing Xorroth must have been a huge blow to them. But if the Legion has a capital world, it's probably Argus.
The Draenei might be looking forward to returning to Argus and healing the planet, cleansing it of the Fel corruption. But Illidan might have a very different plan.
If we wind up destroying Argus, it could cripple the Legion in a very permanent way, or even destroy it. In fact, destroying a planet might be the only way we can truly kill Sargeras.
Could that be our ultimate goal? Are we going to save Azeroth by destroying Argus?