Season 5 of Arrow ended a season-long battle with Prometheus, better known as Adrian Chase. Prometheus could break Oliver in a way that no other villain has been able to do in the past — by forcing Oliver to admit that part of the reason he started his crusade was that he enjoyed killing. Adrian Chase pushed Oliver to the point where he needed to recruit past villains such as Deathstroke, Nyssa al Guhl, Malcolm Merlyn and Digger Harkness to help him. The impression that Adrian Chase will leave on Oliver and the team will have a lasting effect moving forward. Season 5 also saw the development of different storylines that could lead to future threats.
Felicity had an interesting storyline during this season. Starting the season dating Tyler Ritter’s character, Billy Malone, Oliver was manipulated by Prometheus into killing Billy, sending Felicity down a very dark path for most of the season. Felicity joined a group of hackers called Helix. Helix could supply Felicity with data and tools that helped the team with different issues. With every “favor” asked by Felicity, she was asked to help Helix with their agenda in return, even if it wasn't always legal.
While Felicity worked with Helix, the team of hackers learned everything about Team Arrow. This could prove to be a very dangerous power move for Helix now that their leader, Cayden James, is free from A.R.G.U.S. custody (Felicity helped Cayden escape). This fact could lead to Team Arrow trying to reverse the mistake that Felicity made and cause a battle to start between to the two teams.
Black Siren (Earth-2 Laurel Lance) was recruited by Prometheus to help catch Team Arrow off guard and to aid in the mental destruction of Oliver Queen. Because of the impact Earth-1 Laurel had on the team, recruiting Black Siren was an effective strategy. Now that Prometheus is gone, what’s left for Black Siren?
She has already attempted to trick them into believing that she is Earth-1 Laurel and has battled with the teamed several times at this point. With the arrival of Dinah Drake, it is fair to argue that Black Siren has met her match, or that Dinah is the better of the canaries. It could prove to be difficult for Black Siren to become the main villain of the season. Could she have more up her sleeve?
Oliver will have his hands full with trying to find his team after Adrian’s death triggered bombs to explode all over the Lian Yu. Who will survive the explosion on the island? How will they return home? These are all important questions, but one of the most important questions will be about who is waiting on Oliver when he returns.