According to Manu Bennett (who I love for his portrayal of Deathstroke, and Crixus on the excellent Spartacus) Arrow’s showrunners have told him that they definitely want him back. While he doesn’t know the exact timetable, and he may not make an appearance in the second season, it seems pretty well assured that we’re going to see Deathstroke again at some stage.
Manu was one of the highlights of the last season. His haunting portrayal of Deathstroke, and his quest for revenge against Oliver Queen was one of the most epic storylines we’ve ever seen in a comic book based television show.
The focus on charter drama, and development really had Marnie Bennett on the edge of her seat the entire time.