In a panel at the "Amazing Las Vegas Comic-Con" John Barrowman confirmed that he will reprise the role of the role of The Dark Archer/Malcolm Merlyn (Ras Al Ghul ?) in Arrow Season 4. He has this to say about the fourth season of Arrow.
"We start filming in July, season four - I just finalized my deal about a week ago. I'm
very exited about going back, I can't tell you anything becuase I do know abit about what's going to happen. But if you love Malcolm Merlyn, if you love Oliver Queen, if you love Thea Queen, if you love the Canary sister, and I say that with a bit of a twist and a joke in it, your going to love what's going to happen across the DC world"
This quote seems to confirm that Merlyn will appear in the first episode of Arrow Season 4. Prehaps Merlyn will enlist Oliver's help against Damian Dahrk ? What he means by a twist and joke is anyone's guess, but I think he is referring to Sara Lance's ressurection, as she is confirmed to be a regular in Legends Of Tommorow.