Arrow: 7 Plot Threads The Show Totally Abandoned
4. The Ninth Circle
Arrow's seventh season was one that was full of promise but it struggled to make sense of its multiple villains, failing to utilise Ricardo Diaz as well as they had in Season 6 while completely wasting the sibling rivalry between Oliver Queen and Emiko Adachi.
Following a half-season of animosity, Emiko underwent the most truncated redemption arc of the show's history in the finale as she found herself opposed by the Ninth Circle - the faction that she was in control of. And, in the end, the Circle's new apparent leader Beatrice was responsible for her ultimate death - suggesting that there was more to come from the evil organisation.
Given that the Arrow writers knew Season 8 was going to be its last, it seemed like a waste to even bother setting up a new Ninth Circle because with the build towards Crisis (and Arrow's conclusion), it never stood a chance. And even if it did, those chances were eviscerated by Crisis On Infinite Earths when the timeline was reset and Emiko never became the villain that we saw in Season 7.