Considering Ballos' presence, or at least the crown's presence, at some point, somebody else must have come to the island with similar intentions likewise to Date. Have this person be inspired by the actions Date/Ballos commit, have this character attempt to justify Date's actions no matter how outlandish, have the other characters in the story see him as a bumbling idiot at first but as the game progresses when this character starts gaining powers and have it be from a possibly weaker demon crown. Let's assume for a moment Ballos had a backup plan and intends not to use Misery, but instead his own sister Jenka as a slave, hoping that this enslavement will prevent her from doing anything to stop him, you can make up the second slave but make sure they're an established character and has a foreboding presence like possibly the Cthulhu guys, if you want to go the fictional route with an entirely new character, make sure they're written and developed well. With this contingency plan being put into effect, this puts the new villain in the forefront as the new crown holder, and thus can wreck havoc once he attains told power from the crown say 70-80% through the game, and have his build up of powers leave him stunned in bewilderment and amazement whenever he discovers a new power or uses one successfully.
As the game drags on, begin to give this character a gradual deterioration of his sanity and once the crown fully takes him over and allows full access of the new powers he can go insane and basically morph into the new doctor from there, possibly having several encounters with him being both the puppet master and the puppet since Ballos is dead, the reason the crown exists despite Ballos' death is up to you, I wouldn't know myself how to do it without making up some excuse like "the crown's power pursues long after death", think it hard through and make sure it's a good idea. You don't have to use my idea if you feel it's too bland, what I have down is a rough outline, if you want some elaboration please contact me.