Something big always happens in the final episode of Arrow, before they go on a break and return with the new episodes in January. What do people think is going to happen in this years final episode? In the first seasons episode 9, we learned that Malcolm was the Dark Archer. In season 2 we learned that Slade was in Starling City, Blood was working for him, that he and Oliver had come into conflict and he lost his eye and that he was out to get revenge on Oliver. Shado was also killed in that episode and Slade was injected with Mirakuru and got his powers. In season three, we found out that Thea killed Sara and that Malcolm was behind it and we also had the epic fight between Ra's and Oliver with Ra's beating Oliver comprehensively.
Personally I think we will learn more about Darhk's main plan. Perhaps we will get to see or hear about his associates. I think they will be revealed later on in the season (maybe one or two will be revealed early though).