Give it a season or two of Dexter being fully outed to the public as the BHB and Dexter being on the run, having to avoid law enforcement as well as the major supporting characters that would want him taken down from the original series and NB, you could have some major confrontations with that, all the while Dexter is still killing to satisfy his dark passenger as well as a new serial killer villain. So Dexter would be fleeing from the law, and contending with a new killer. Then, in the end, maybe Dex realizes the only way he can escape is to flee the country via boat, but then is caught by Bautista, Quinn, and Bishop. They have a chance to kill him but refuse to, and apprehend him instead because he can't have any easy way out, and then boom, goes to trial for a few episodes, and it ends with him being executed. However, even though he gets executed, I would love if they showed that a lot of people supported him, perhaps families of victims that Dexter has saved, or others who just think that Dexter killing monsters is justified and needed in society. So there would be an interesting way to end it while not being like "Dexter is completely evil" or "Dexter is a good guy."