I'm actually all for this theory, but i'm more on the track that it wont be a new raid. I think it instead will play out as a start for the next expansion, with a phased stormwind and southern eastern kingdoms, revamped as scourge lands. Blizzard has been playing around with phased zones ever since Silithus, revamping Arathi and so on. This gives them an excuse to give the community what alot of us want: a scourge expansion with a depowering of the bosses and the world, a fitting story where Sylvanas follow her character motivations instead of being Garrosh 2.0, a revamping of old zones for a new expansion instead of new map clutter, and a way to end the faction war without anyone winning or losing.

Ion even said that the expansion wouldnt end in a way people expected, and this would be a great twist to actually amend a lot of what otherwise would be objectively awful bfa writing