In an interview with executive producer Marc Guggenheim, E! Online found out that Tommy Merlyn (Colin Donnell) will make a third postmortem appearance on Arrow. Since his death at the end of season one, the younger Merlyn has appeared as a hallucination in season two and in a flashback in season three.
When asked about bringing the character back Guggenheim said:
We love Colin Donnell and we love having him on the show. I was on set for his scene at the end of season one and the second we wrapped him, I said to him, "I'm so glad we do flashbacks on this show!" So yes, you have not seen the last of Tommy Merlyn.
Though I will say this, the next time you see Tommy Merlyn, you will see him again on the show but it won't be the way you're expecting.
So how will Tommy come back? Since both of them were dead at the end of season one… maybe Malcolm and Tommy were both taken to a Lazarus Pit.