Dont you know? They already had the Jailer planned since WC3 when Arthas became a death knight, is the culmination of 20 years of story telling after all.
Of course that 1 line of dialog in Legion, exquisite story telling that let the reader know what was coming 2 expansion after, we all knew Sylvannas was working for the Jailer and the whole Shadowlands was already conceive. Such rich lore, with 0 retcons.
It could have been easily been any Old god or even a mad titan, but no it was clearly the Jailer all along. All planned since WC3.