According to Deadline, John is slated to film for a couple of days for the Dexter reboot, so it’s unlikely that the Trinity Killer will turn out to be alive and well. Instead, one possibility is that the Trinity Killer will act as an imaginary guide for Dexter on whatever new life he has under an assumed name.
The original series ended with Dexter living up north, away from Miami and, presumably, having lost contact with everyone from his former life.
The title character has always talked to his dead father, though, who serves as a sort of mentor. Dexter has always imagined his father in the same room with him to help him through his murderous deeds and, because the Trinity Killer is also an important male figure from his past, he too could become a guide for Dexter, at least for an episode or two.
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