I don't think there will be a spinoff, I think that based on the reaction the final season got, Showtime wouldn't want to take the gamble.
I mean, I'm still trying to understand how this last season could be so horrible. Say what you will about last season, but at least it had a solid final episode.
For a show that got this popular, you'd think they'd have spent more time/money on it to ensure quality. Old photographs on the show look like they were taken yesterday on a new digital camera...CG Hurricane fails...rubber babies....CGI snakes...horrible stunt doubles....horrible fake websites/PC programs. Just all around cheap crappy props.
Then, who the hell checked for plot holes and inconsistencies? When you establish that Dexter is a "neat monster" and adheres to a strict code, you can't just have him kill innocent people with no explanation as to how he excuses himself and you can't have him getting sloppy while tracking people (standing in windows in plain sight, parking right next to the victim, getting caught on camera, etc)
Dexter was great for the first 2 seasons, it lost it's way in Season 3 and then came back to redeem itself in Season 4. It became a show riddled with rookie mistakes and bad writing.
Also, since people mentioned Debra's character earlier..... The stupid writers saw that Debra was gaining a cult following for her cursing and Masuka for his creepy sexual comments, so they just decided to go completely over-the-top with it and make that their prime focus on those characters. They made you start to like Batista and Quinn only to make them useless background characters by the end.
They could've had Hannah try to kill Dexter this season and have Lumen come back and kill Hannah, I mean they left the door open for something like that. It would've been a better ending than what we got.