Now we all know Tommy Merlyn is infact dead and it was a crucial event to make oliver into the hero he needed to be, And besides a couple of flash backs and references we have heard nothing about him. However in the fourth season His father Malcom having made up with team Arrow, made a deal with oliver to become the next Ra's Al Ghul and at the end of the season he actuall did become Ra's.
In the comics and the show Ra's has a magical pit called the lazarus pit that can heal injuries or return people from the dead. We have already seen this in action with Thea in arrow and Sarah Lance in the new show Legends of Tomorrow. So obviously it can happen, to Tommy as well. However what would be the purpose?
1. Malcom felt bad.
2. To adapt Tommy into a new persona.
I really hope they bring tommy back and into a new persona as the red hood, yep the red hood.
I mean it would make sense, in Season one oliver is known as the hood. Also How badass would it be to adapt the under the red hood stroylin into cw verse. In summary malcom as the new Ra's revives Tommy into a new persona. (sorry for the bad grammar and minimal writing I just wanted to get this out there)