Revendreth: Denathrius already gave him his: check
Bastion: Anduin got Kyrestia’s key: check
Ardenweald: the key is probably in the Heart of the Forest: to be obtained
Maldraxxus: “do not let the Jailer reach the sepulcher”: the key is hidden in the Sepulcher of knowledge: to be obtained
The Arbiter IS the final key
What do you think the keys are for? Are they for the alleged secret?
What do you think the secret is?
Also, in Bellular’s QandA, in the context of the morality answer, they said the titans and Order were benevolent to those who agreed with them and we would yet see what they did to those who disagreed with them. I think they are referring to the Jailer.
I think Denathrius Key might still be with him or his sword which gives the Writers an easy excuse to bring him back in whatever way. Considering Anduin could just waltz into Bastion and attack the Archon in her own Fortress there is nothing stoping the Jailer and his friends from just taking back Denathrius.
With Maldraxxus i´m on the fence between Draka somehow having the key considering they refer to her as such in her Cinematic and her dying makes perfect sense to develop Thralls story further or the key being locked either in the sepulcher or the sword.