Thank you, thank you. But you see, it was actually quite easy to predict all this stuff. Since alpha datamining, we knew that the Janitor was after some "key" carried by the "Arbiter" that would allow him to reach a certain "Sepulcher". Blizzard, being the masters of subtle foreshadowing that they are, literally had the Primus say this, and this dialogue was datamined since alpha:
https://wowpedia.fandom.com/wiki/The..._Unknown#NotesVoice of the Primus says: There is but one hope to save the Shadowlands. The Eternal Ones must stand together once more, before the Jailer escapes the Maw.
Voice of the Primus says: Bring my warning to the Archon, the Winter Queen, and the Sire. They must see to our defenses. Do not let Zovaal reach the sepulcher.
Voice of the Primus says: The Arbiter is the final key. Protect her, or all is lost.
My only "mistake", if you can even call it that, was assume that Shadowlands would be a 3 tier expansions like literally every other expansion except for WoD. In which case, you can be certain that Kel'thuzad would have had his own raid tier (he was the most popular villain featured in the expansion besides Sylvanas).
Indeed, one of the main criticism of Shadowlands (amongst the countless of them) is that Ner'zhul and Kel'thuzad deserved more spotlight.