As with any series, the original 8 seasons of Dexter had some missed opportunities, but John Lithgow's casting in the upcoming reboot can fix one of them. Michael C. Hall first hit the screen as Dexter Morgan in 2006, when the Showtime dramedy introduced the titular serial killer who satisfied his urges by targeting other murderers. While growing up, he found a mentor in his adoptive father, Harry. He was the one who first introduced Dexter to a moral code about who was "rightfully" targetable and who wasn't.
Over the course of the show, Dexter garnered a slew of both allies and enemies — from the Ice Truck Killer (who was also his brother, Brian), to Zach Hamilton, to Hannah McKay. His season 4 adversary was Arthur Mitchell (John Lithgow). Also known as the Trinity Killer, he was a well-received villain for the show (with the actor even receiving a Golden Globe and Emmy for his role). His character, who also started killing in his youth, was a solid, equally tragic parallel for Dexter: he was driven to a penchant for murder by a seeming hybrid of genetics and searingly painful childhood experiences that seemed to ignite the predisposition.
There was only so much that Dexter could do with Mitchell's character, who died at the season's end. This is unfortunate since Lithgow's performance in the role was a series highpoint. It appears that Dexter has found a way to correct this missed opportunity: John Lithgow appearing in Dexter season 9. Although details surrounding his role are limited, sources indicate it will be a small, yet meaningful, appearance. Although the Trinity Killer may be used for a flashback, it would be wise for Dexter to instead feature him "haunting" Dexter — something the original series should have done.
Dexter could have really benefited from its titular killer being "haunted" by Arthur — after all, a vital component to Dexter's character and story was that he was tortured by his past and those he'd lost along the way. He'd already internalized versions of Harry and Brian, who he would often converse with while wrestling with his Dark Passenger (the name Dexter gave his compulsion to kill). Mitchell, in many ways, is Dexter's equal: he's another serial killer grappling with his own demons and trying to make sense of his place in the world. He was an understanding friend for Dexter, who could also offer genuine empathy for his situation. Of course, saying the relationship, and Mitchell's life, ended on poor terms would be an immense understatement. Dexter killed him, and it was quickly revealed that Mitchell had murdered Rita beforehand. That's just another possible, dark reason that Dexter could be haunted by the old foe.
Mitchell's return can also help the revival drive home its implicit message about the futility of fighting one's Dark Passenger. Though Dexter tried to completely abandon his old life at the end of season 8, leaving Harrison behind for an entirely new existence that would be better for both of them, the Dexter reboot's teasers suggest his compulsion to kill is inescapable. Before his death, Mitchell himself told Dexter that he had never been able to fight off his homicidal urges. He even told Dexter that he believed God had sent the show's titular killer to take him out. While on the kill table, he warned, "You can't control the demon inside of you any more than I could control mine." And now, with season 9 approaching, it's looking like those words ring true — and Lithgow's return can help flesh that sentiment out.