Meteion was slightly hinted at especially in Shadowbringers with the Final Days. We just didn't know what the cause was. In Crystal Tower there's also morse code that decrypts to something she oftens says: "It's all wrong!". Ever since the end of 1.0 and ARR, we knew the Ascians wanted to bring back Zodiark we just didn't know why. In general there was a natural progression to her.
I used to play WoW so I can speak on this. Back in Legion, Vol'jin chose Sylvanas to take over the Horde with his dying breath. Nobody knew why except the spirits told him. In BFA then the troll god of death started mentioning how she had a secret boss. And she was acting erratic, but there were hints she was serving someone else.
In Shadowlands that's revealed to be the Jailer. There are a few issues with the Jailer though. The first is that he's retroactively retconned to have secretly been manipulating events thoughout history the entire time. That he was secretly behind the Legion, the Scourge, etc. But there's never been any hints of this ever. WoW up until WOTLK was pretty much following off of Warcraft 3's storyline, and in WOTLK when you fight the Scourge there's no hints of the Jailer at all. Now they're retroactively acting like The Lich King Arthas was trying to defend Azeroth against the Jailer even though he never mentioned anything of the sort. They're telling us the Nathrezim secretly served and manipulated the Legion and its Demon Lords the entire time, even though again there's no hints of it. Even the Appendices of the Warcraft 3 manual, which is what Blizzard had to dig into so they didn't have to finish off the Legion storyline for three expansions after WOTLK, nothing about the Shadowlands or the Jailer or First Ones etc is ever mentioned. The problem with WoW's story is after WOTLK it started to become more centered around its player retention model, which is to drag things out as long as possible and timegate everything. WoW's other big issues is they constantly retcon things to suit their needs. I could go on for days about WoW and Blizzards fall from grace but that's not what the topic is about.