This is a bit of me tongue in Cheek,
But Id like to see the option in the last or semi final last scenario for Season 4, to have an option for a character on the table to become a Darklord and be removed from the campaign forever. Now I cant really fathom a reason how this would happen... perhaps a character has to kill a lot of innocents to convince the Dark Powers to let the others get out but I think it would be awesome.
In earlier editions, characters could be tempted by the "Dark Embrace", which enticed characters to commit ever increasing acts of evil in order to receive a blessing or boon. Stage 1 was extremely easy to succumb to, only requiring a minor act of evil (lying, cheating, deceit, etc), while the last stages (stages 4 and stages 5) required heinous acts in order to draw the attention of the Dark Powers. Each stage was accompanied by a saving throw (similar to a madness save) representing whether the Dark Powers took notice of the act and granted you a boon.
By Stage 3, characters would start showing physical manifestations of their Embrace, misshapen limbs, bulging/cat-like/different colored eyes, scales, fur, etc. Each stage also granted increasingly more powerful boons. Upon accepting Stage 4 of the embrace, the character becomes a villainous NPC under the control of the DM, forever bound to the demiplane (but free to travel to different domains within). They usually undergo a dramatic transformation at this point while the Dark Powers cloak them in protective magics that make it difficult for their true nature to be revealed.
At stage 5, the next time the character enters the mists, they part to reveal a new domain (which can be as small as a single room or structure, or as large as a country), and the character becomes a Darklord of that domain. Stage 5 was always subject to DM fiat, as the character had already become an NPC at stage 4.
I personally would love to see the Dark Embrace return, however it should be known that (assuming the stages remain the same), characters would be forcibly retired upon reaching Stage 4 (or equivalent). In fact, it is entirely possible they might be forced to retire prior to that (esp. if alignment shifts to NE or CE occur at Stage 3).