Many people complain that the Jailer is a completely dull and boring villain. And it is true. So how would you fix that? How would you make it even a little interesting?
Personally, I would just make the Jailer right. If all the Covenants (we will not touch on the other endless realms of the Shadowlands) had some kind of huge flaw that makes mortals suffer (we clearly see this in Bastion, mortals are forced to give up their memories and some of them become Forsworn) and which the Jailer would like fix, for which the Eternal Ones (who could not see the righteousness in his words and the imperfection of their own ways) drove him out. After the ages of imprisonment in the Maw, he could become much angrier, but the only thing he would like is to take revenge on the Eternal Ones for unfair exile and correct their Covenants. No desire for absolute power.