Well I could see them going for the next season with Deb wanting to think it is the only time he has killed anyone like that (trying to protect Dexter and maybe even blaming herself for exposing Dexter to his brother Rudy? LOL) but being very suspicious of him....
her trying to live in denial but feeling more and more that he is a killer.
And of course I expect she'll never see him romantically again! I mean Harry knew that what he witnessed was Dexter's first time, and it lead to his suicide!

I'm guessing the final season will be Dexter on the run/tried for multiple murder/killing people in jail/death penalty....