The original ending that the writers and producers had in mind for Arrow was quite different from what actually happened in the series finale. The first Arrowverse series concluded its eight-season run with a funeral for Oliver Queen (Stephen Amell), who died in Crisis on Infinite Earths, followed by a sweet reunion between Oliver and Felicity; however, the show's creator originally had a different vision for how the show's final scene would play out.
Arrow began building toward Oliver’s death when Oliver had to make a deal with the Monitor (LaMonica Garrett) to save the Flash (Grant Gustin) and Supergirl (Melissa Benoist) in the 2018 Arrowverse crossover event, Elseworlds. It was revealed in the season 7 finale that the cost of making a bargain with the Monitor would be his life. Season 8 focused on Oliver keeping his word to the Monitor and going on one last mission to save the multiverse from the "Crisis." After becoming the Spectre, Oliver defeated the evil Anti-Monitor and managed to use his power to rebuild the multiverse and create Earth-Prime. In the Arrow series finale, Team Arrow paid their respects to Oliver. In the final scene of the scene, Oliver reunited with Felicity (Emily Bett Rickards) in the afterlife.
How does this differ from the original plan for Arrow’s ending? In an interview from 2012 (before Arrow season 1 even premiered), Arrowverse executive producer Marc Guggenheim laid out the show’s flashback formula and explained that the last episode would show Oliver getting rescued [via EW]. The idea behind this was that Arrow was intended to be an origin story, so it made sense for Arrow to end with the final flashback of Oliver’s five-year absence from Starling City. Obviously, this isn’t what happened. Arrow did indeed include the scene of Oliver’s rescue, but that ended up being incorporated into the season 5 finale, well before the end of his Arrowverse story.
The reason why this changed is because Arrow outgrew its original plan. Arrow completed Oliver’s origin story in five seasons. Each seasoned consisted of a year’s worth of flashbacks. Since Oliver was only gone for five years, there was nothing left to say about Oliver’s past after season 5. It wouldn’t have worked for this scene to be saved for the season 8 finale. Oliver’s journey lasted so long that it allowed him to accomplish things that went beyond the original scope of Arrow. For example, it was stated that Oliver’s mission to save Star City was finally over in season 7 rather than season 8, which viewers would normally expect to be how the show should end.
Arrow became something so much bigger than what it was originally envisioned to be. First of all, Arrow was always going to be a grounded show, but as the world around it grew, Arrow changed and evolved as well. Arrow expanded and created an entire shared universe and shepherded the rise of key DC Comics superheroes like the Flash. It also used magic and metahumans. None of these things were planned from the beginning. Similarly, Oliver giving his life to save the multiverse was an event that came as a natural result of Oliver’s growth as a character and the direction of the Arrowverse.