No one else posted this yet and news of season 7 has already coming out, a lot of this can be verified on spoilertv.com:
Season premiere is titled "Are You..." and will include a flashback to Deb and Dex as kids. According to the casting call the ages are Dex at 9 and Deb at 5... so at the very least Dex has started killing animals. Also I'm pretty sure the producers have said there's no time lapse between the seasons.
Ray Stevenson has been cast as a major recurring role as a big shot in an Eastern European crime family. Apparently he's in Miami to find who killed an associate of his. It is not known if he's the big bad... but it seems like the season will revolve around organized crime.
They're casting for a recurring role that's supposed to run through 2 seasons: A woman named Hannah who could be a potential gal pal for Dexter... also might have her own dark passenger.