Maybe the biggest WoW bombshell at Blizzcon this year was the announcement that in a patch after 7.2, but still apparently during the Legion expansion, we're going to be heading to Argus.
In case you don't know what that is: Argus was the homeworld of the Eredar people. Sargeras came there and offered infinite knowledge to the curious, intellectual people, and two of the Eredar leaders, Archimonde and Kil'jaeden, accepted the offer while Velen had a vision that showed what the offer truly entailed. Velen convinced a number of Eredar not to take up the offer, but in so doing, incurred the wrath of those who had taken Sargeras' offer. Velen and his people fled, renaming themselves "Draenei," meaning "Exiled Ones."
As far as we can tell, Argus is more or less the home world of the Burning Legion. Sargeras is its leader, but he's about the size of a planet. The Eredar, however, were given a prime position within the Legion's hierarchy, and Archimonde and Kil'jaeden have served as Sargeras' two top lieutenants.
If you play a Draenei character, there's a good chance you were born on Argus, and you have waited thousands of years (originally they said 25,000, though it seems as if they've revised that down to a mere 13,000) to get home.
In terms of geography, there's really only one major feature we know of Argus, which is the capital city of Mac'aree. The Eredar pre-Legion were already an ancient people, and had fused art, science, and magic to create essentially a utopian society. Eredar seem to live pretty much indefinitely, as Velen seems to be the only one we've seen who actually shows signs of old age. I would assume that if their species was not simply long-lived thanks to their inherent biology, then perhaps they used magic or science to alter their aging process or remove it entirely (perhaps Velen looks old because he had already started to age before these scientific advancements were developed, which would really make him that old. Either that or, like US Presidents, having a stressful job leading your people will age you a lot faster.)
Mac'aree was a super-advanced city that I would guess would make Suramar look like a ramshackle slum by comparison. However, given that it has also presumably been the very heart of the Burning Legion for all this time, we can probably assume that it is a dystopian megalopolis of horrors by now.
In terms of gameplay, I'd be surprised if it's really anything more than a new zone. We'll probably arrive at some sort of battlefront where we'll push against the Legion and work our way toward the expansion's final raid (assuming that we get the standard three tiers.)
We're probably going to link up with the Army of the Light, which is composed of survivors from world that the Legion destroyed. At the moment, it appears that Turalyon is either its leader or one of its leaders. I'm curious to see what other races are introduced, but I have a theory about what sorts of fighters we're going to see:
So, there's a spoiler here for the Paladin class campaign. During your final quest in the campaign, you get reinforcements from the Army of the Light, specifically a Paladin named Lothraxion, who becomes a class Champion.
Lothraxion appears to be a non-demonic Nathrezim, aka Dreadlord.
Given that one of the best examples of long-term enemies of the Legion is the Draenei, it would make sense for much of the Army of the Light to in fact be uncorrupted versions of familiar demons. It appears that most demonic species are those who acquiesced to the Legion - we've even seen it on Azeroth, where Night Elves have become Satyrs. So wouldn't it make a lot of sense for the Army of the Light to be a bunch of Succubi, Felguards, Imps, Inquisitors, Jailers, Observers, and Infernals in uncorrupted forms?
The Vengeance Demon Hunter artifact weapons are the Aldrachi Warblades, but we don't really know anything about the Aldrachi other than that they fought the Legion and lost (like everyone has.) Now, it's probably more likely that we're just not going to see the Aldrachi, but what if we already have? What if, say, the Felguards were originally called the Aldrachi? And we've only seen the corrupted version of them. After all, we had seen the Man'ari Eredar long before we encountered the Draenei.