I think those two are way too on-the-nose. Quinto's most famous role is a serial killer and Emerson was recruited to Lost after playing a recurring one on The Practice.
Anyway, this is what Season 5 made me think of:
Fan-fiction.
Specifically, fan-fiction written by a teenage girl.
It wasn't bad fan-fiction, and I may not have even noticed the signs if I wasn't so immersed in the internet and TV Tropes for years, but once I did(around the 9th or 10th episode), it significantly coloured the season for me.
Rita is suddenly and shockingly killed off and the kids move out to their grandparents'. Dexter is alone again. He rescues a young woman from a group of white-collar rapists, who insists on being a part of his life. She couldn't take her bland, white-bread home life and parents who wanted her to marry a bland college guy anymore, so she ran away. Dexter is initially aloof to these advances because he's Dexter, but comes to accept her because he sees she understands him. She doesn't think he's a monster, but a hero. She gets his "darkness", and even shares it. She won't screw up and go too far like Lila and Miguel. Dexter decides to make her his partner. He gets her a set of black leather gloves like his own(which she reacts to like they're iconic) and teaches her terminology used officially among the show's fans, but usually just informally in his head on the actual show and novels("....what do you call it again?" "A Kill Room"). Eventually, after she gets to personally kill a target, they sleep together and become a couple.
If I read that summary, I would automatically assume it was one of the countless Dexter fanfics that are probably out there, and that the author was a possibly-emo teenage girl. That fact that it still managed to work pretty well and reach what I feel was a moving, if not entirely satisfying, emotional climax by the finale is testament to the skill of the writers. It was certainly a surprising and risky direction to go in, but I guess the show has always had aspects of wish-fulfillment as part of its appeal. Again, a lot of this is just going to ride on where it goes next....