C'thun being revived by the Azerite shower he got, and he decides that the sword is the best stronghold for his body this time, so he starts burrowing inside it instead of the planet, with his big eye being where the big "energy ball" was before. Basically, the sword itself becomes a monument to him, where he watches over his new city/empire.
Imagine all of Silithus and Ahn'qiraj being reworked into one large, spiraling temple city, with the sword at its center and C'thun living in it, watching over it all. Now that would make an OG feel like a big deal again, just by a visual point of view, and actually give a feel of "constantly being watched" while playing there - because you actually are being watched.
Would be a pretty sick endgame zone imho in the spirit of Suramar: you fight yourself through the outer reaches of the city while leveling, and then the inner core+sword are a raid.
And it would be a little callback to both the swords original design (where an eye was at that spot instead of an energy orb) and the old, deleted Cata questline which was supposed to end with C'thuns gigantic eye floating over Silithus.
Granted, it would be veeeeeery similar to Sauron in the LotR movies lawl, but...basically everything Blizzard does is ripped from somewhere, so might as well go all out with it.