Much as people discovered an "Iron Horde" texture in Mists of Pandaria's 5.3 Insurrection (or whatever the pre-Siege patch was) patch, which gave us our first obscure hint of the Warlords of Draenor, much buzz is coming up around a few textures and icons that are labeled Kul Tiras, with what looks like an armor set that incorporates a lot of nautical-themed elements like ropes and anchors, as well as a rough in-game (as opposed to UI-based) map.
Given how Kul Tiras has nothing to do with Argus and that even more than the Iron Horde texture (which was just kind of a grey square) these are very clearly referring to stuff that don't seem linked to Legion, it seems very hard to come up with any explanation other than that Kul Tiras will play a role in the expansion that follows Legion.
Let's talk Kul Tiras lore:
Kul Tiras was one of the seven human kingdoms, along with Stromgarde, Lordaeron, Gilneas, Alterac, Dalaran, and Stormwind. Kul Tiras was the major naval power among the kingdoms, as it was situated on an island off the coast of the Eastern Kingdoms (not far from Tol Barad.) The monarch of Kul Tiras is called the Grand Admiral, which gives you a sense of how important sailing is to their culture. The previous Grand Admiral was Daelin Proudmoore, father of Jaina.
As Jaina was establishing Theramore down the coast from Orgrimmar, she had worked to secure a lasting peace with the newly-situated Horde. Her father, unwilling to make peace with the Alliance's old enemy, led his fleet in an attack on Orgrimmar. Jaina ultimately allowed Rexxar to lead Horde forces into Theramore to defeat and kill her own father in exchange for a lasting peace - which is why the notion that Jaina is overreacting in her current antipathy to the Horde is total absurdity. She literally sacrificed her father for peace, and was paid with the destruction of her city in return.
Since Cataclysm, Kul Tiras has been missing. While it never appeared in-game, the notion was that it was still where it had always been. However, as Tol Barad was added in that expansion, the fate of Kul Tiras remained a mystery, and devs suggested that it had been moved by the massive tectonic shifts that came with Deathwing's emergence from Deepholme.
While not officially confirmed until we get a real announcement, I strongly believe that Kul Tiras will be a location in the next expansion. The question, then, is what form it will take: A zone, or the whole expansion-spanning continent.
I'm leaning toward the former, as I think that a sea-based expansion would benefit from having other island cultures, such as a visit to Zandalar, perhaps a return to Kezan, and perhaps a journey to Nazj'atar.
To return to lore for a moment: in the apocryphal WoW tabletop RPG, Jaina had brothers, one of whom could now lead as the Grand Admiral of Kul Tiras. If these brothers are not, in fact, canonical, it would mean that technically Jaina should be the Grand Admiral and Queen of her people. Jaina has been notably absent from Legion, and it seems that trouble at home would be the only reasonable explanation.
Now, I could actually imagine Kul Tiras as a continent, even if I think it would work better as a zone. Taking the Broken Isles as an example, in ancient times, the zones of Suramar, Azsuna, the Broken Shore, and Val'sharah were all considered sort of "the Greater Suramar metropolitan area." We see this with Lordaeron and Stormwind as well, each kingdom being represented by a cluster of zones rather than a single one.
Still, given the nautical theme of Kul Tiras, I would find it very hard to imagine an ocean-faring-based expansion that does not include the aforementioned zones - particularly Nazj'atar, as I can't imagine a sea-based expansion that does not put the Naga at the forefront.
I wonder if they'll keep the level-scaling of Legion. I think it worked out well, with the one flaw being that the story couldn't really develop zone-to-zone as you leveled up, and that if you picked your zones in the wrong order, you could feel handicapped (like if you got a class quest sending you to Azsuna right after you'd chosen Stormheim as the zone you were heading to next.) Certainly it helped provide max-level content - I'm happy to have now six zones to do world quests in instead of being stuck with only Suramar and Broken Shore (even if Suramar might be my favorite zone in the expansion.)
We don't have a trademark for an expansion title, which I think will give us a much clearer idea of what the expansion will be, but we can infer a huge amount of information from the presence of Kul Tiras items. If you're feeling skeptical about this, I would consider that the quality of these textures is really way too much to dismiss them as meaningless. Aside from an epilogue-patch trip to Kul Tiras (which again, seems to have utterly nothing to do with Argus and the Legion) that seems incredibly unlikely (I don't think we've ever gotten a whole new zone after the final raid tier; the biggest thing we ever got as an "epilogue" patch was a one-boss raid in the Ruby Sanctum,) the only other explanation I could give you for this is a setting for a new expansion.
Gamescom, which is where they announced Legion two years ago, runs August 22nd-26th, and Blizzcon runs November 3rd-4th. I wouldn't be shocked if they announced the next expansion at Gamescom to keep up the sense that there's something coming, though 7.3 is unlikely to go live until the end of summer or the beginning of fall. Before Mists, new expansions were always announced before the final patch of the current one came out, and while I can tell you I'm happy to focus on Argus right now (holy crap am I excited for Argus) I also think it's wise for Blizzard to always keep players aware of what is coming next with WoW. The Siege of Orgrimmar and even more Hellfire Citadel patches seemed to go on for an eternity largely because for a big stretch of both, we had no idea what was coming next. Giving players something to speculate about and await with eager anticipation is never a bad idea.