Warning: This article contains SPOILERS for the Arrow season 6 finale.
Arrow broke all of its own rules in the ending to the season 6 finale, "Life Sentence." Normally, a season of Arrow ends with Oliver and Team Arrow defeating the bad guy and getting a bittersweet ending, if not a completely happily ever after. In "Life Sentence," only one usual trope, the defeat of the villain, happened... sort of.
In order to take down big bad Ricardo Diaz, Oliver made a deal with Agent Samanda Watson and the FBI. With Watson's help, Oliver was able to run the gangster out of town, take out Diaz's army, and uncover his secret network of contacts. The only hitch is that Diaz survived. Due to a botched murder attempt by Black Siren, Diaz escaped Star City with his life. Yet, this isn't even the worst thing to happened to Oliver.
Oliver Queen ended season 6 behind the bars of a federal supermax prison. Oliver's deal with the FBI came with some heavy strings attached. After trying to arrest Oliver all season and expose him as the Green Arrow, Agent Samanda Watson finally got her wish. Oliver exchanged his freedom for the takedown of Diaz. Oliver secured immunity for the rest of his vigilante family but didn't get any of his own. According to the terms of his sentence, Oliver will spend the rest of his life in prison.
The season 6 finale was evenly split between Oliver making amends with his fractured team - excluding Curtis, for some reason - and trying to neutralize Diaz. Oliver buried the hatchet with Dinah, Rene, and Diggle. The scenes even managed to salvage the terribly misguided storyline of the Team Arrow civil war. Throughout the course of Oliver's farewell tour, he even had to say a very unexpected and tear-filled goodbye.
During the climax of the action-packed finale, Quentin Lance was shot by Ricardo Diaz, trying to protect his "daughter," Earth-2 Laurel Lance. The wound ended up being fatal. While in surgery, Quentin seized and later passed away from lack of oxygen. Before Quentin expired, though, Oliver was allowed to visit Quentin's bedside. Oliver explained that he learned to become a father from watching Quentin, not his own father. Quentin's death was sudden but not too shocking as Paul Blackthorne's exit as series regular had been announced before the season finale aired. Arrow still managed to get the most emotional mileage from Quentin's death, especially since his real daughter Sara Lance arrived too late to say goodbye. (Although, since she has a time machine, that shouldn't have been an issue.)
Of course, the genre defining twist of the finale is Oliver's imprisonment. It'll change the series forever. No matter what happens, Arrow always finds a way to restore the status quo. Even the big season 5 cliffhanger, where the entire cast, besides Oliver, could've been killed, amounted to very little. Samantha Clayton, the mother of Oliver's child, was the only casualty of those events. However, there's no way that Arrow can ignore Oliver's imprisonment.
Obviously, Arrow will have Oliver find a way out of prison. Stephen Amell is the face of the show and he's not going anywhere anytime soon. Yet, with Oliver's secret identity being outed, there's no way Arrow can just brush the ending of season 6 under the rug. When (not if) Oliver gets out, he'll be in more danger than ever before. Every criminal will know where to find him and hurt his loved ones. Arrow season 7 just got a lot more interesting than previously expected.