Sargeras is or was Argus! I posted something similar of this on my Youtube Channel Skoll Shorties. I'm not allowed to link sadly.
Alright, so let me get my points across here I guess. First thread here so if I am not the best I apologize.
Now this idea is pretty far fetched of mine and I can definitly understand the need to point out its flaws, holes, and as to why this isn't possible but I want to at least get the idea around to get people's opinions on it. Okay? Okay.
Sargeras is current Argus : After Sargeras was thrown away from the portal to Azeroth he was thrown into a weaken state after expending his soul into that of an ancient, killing the Titans, and all those years of hunting demons, cleaving planets, and etc. Titans need a nap too, yeah? So Sargeras takes a long needed rest within the very core of Argus. Corrupting the already fel painted world more than any other planet in the Great Dark Beyond - pissing off Kil'jaeden (who is now the speaker for the planet of Argus, similar to Magni is to Azeroth currently). This would also explain why Sargeras was nowhere in sight or near Argus when Gul'dan was doing the ritual to pour Sargeras' soul (His Master) into Illidan's lifeless body during Nighthold - Sargeras was nowhere in sight - but what was? That's right Argus! ...Again pretty up in the air idea and its probably inaccurate, but it would be one boss cinematic to see Sargeras rip from the very world we're standing on and him becoming the very platform we fight on.
Sargeras was Argus: Another stretchy concept of mine that goes hand in hand with my previous paragraph, but hear me out at least. As we know all Titans came from a Planet at one point and they would shortly pull themselves out from said planet to walk amongst their fellow Titans. Sargeras was no exception and would be born from his planet and later would join with his fellow Titans (which he would promptly kill). Argus was that planet that Sargeras was once with as we do not know what happens to the planet after the Titan births from it. It would make sense why Sargeras was so drawn to the planet and why the Eredar were a must for his Legion - the very inhabitants of his planet would prove the most useful in his Legion.
So that is my far fetch idea, probably inaccurate. Probably wrong. Probably gonna get ignored or gonna be told why this isn't gonna work but that's okay. I hope you at least liked this idea and or might agree with it. Thanks for taking the time out of your day to read this or watch my video. Thanks!