A common theme of comic book movies and TV over the last decade or so has been that actions have consequences. One of the central themes of The Dark Knight was that of escalation, positing The Joker as the inevitable consequence of Batman's vigilante justice. More recently over on the Marvel side, the events of Captain America: Civil War were largely a consequence of the destruction wrought during the events of Avengers: Age of Ultron.
This theme has carried over to The CW's Arrowverse as well, not only with Arrow itself but also with its generally lighter-in-tone cousin, The Flash. With The Flash, cause and effect are spelled out in big, neon letters with the particle accelerator explosion of season 1 causing a ripple effect that is compounded not only by time travel but travel to other universes. Arrow has always been a bit more muted in exploring those themes, but they are there nonetheless.
In recent comments to EW, Arrow executive producer Marc Guggenheim explained that season 5 will explore the idea of Oliver Queen's past coming back to haunt him. Some of those past events will already be familiar to viewers of the show:
"Essentially the theme that we’re dealing with this year is legacy. The idea [is] Oliver honoring Black Canary’s legacy after Laurel’s death last year. As the season progresses, you’ll see that there are a lot of callbacks to season 1. What we’re doing is we’re dramatizing Oliver’s desire to grow, move forward, and evolve, but this concept of legacy keeps threatening to pull him back to the early days."
What will the consequences of those early days be? For one thing, the mysterious new villain Prometheus, a character whose existence Guggenheim hints is a result of Oliver's past actions, although how exactly he ties into Oliver's past is still under wraps:
"I can really only say that Oliver bears a measure of responsibility for the creation of Prometheus."
All we've seen so far of Prometheus is a few fleeting images of him in a full mask, with no clue as to the face that might be underneath it. Guggenheim has already stated that he is not the comic book character of the same name, and that "[It’s] a new character, someone who fits into the theme of legacy this year that’s going to be driving all of our characters, including the big bad." Obviously Guggenheim is hitting that legacy theme pretty hard in promoting the upcoming season, but it appears fans will have to wait a while yet for the big reveal on who exactly the Green Arrow's newest arch-enemy archer is.
The Flash season 3 will premiere Tuesday October 4th at 8pm on The CW, Arrow season 5 will premiere in the same timeslot on Wednesday October 5th, Supergirl season 2 on Monday October 10th, and Legends of Tomorrow season 2 on Thursday October 13th.