X-Men Evolution's fifth season (which was never formally produced) would have adapted the Phoenix Saga, but thanks to four years of very specific character growth, the series had the potential to tell a character-driven and heartbreaking version of the story that might have had a better chance at adapting the story's true weight than any other attempt.
X-Men Evolution ran for four seasons, from 2000 until 2003 on Kids' WB. The show was a hard reboot of the X-Men concept, re-imagining many of the franchise's most iconic characters -- Cyclops, Jean Grey, Nightcrawler, Shadowcat and Rogue -- as high school students attending the Xavier Institute. Under the guidance of their mentors (including Charles Xavier, Storm, Wolverine and Beast), the team juggled high school drama with being hidden heroes. The show's fourth season saw the team and their assorted rivals and allies face-off with Apocalypse, ultimately preventing his plan to mutate the majority of the world's population. The series notably ended with Xavier speaking to his students, having glimpsed their futures while in psychic contact with Apocalypse.
The resulting montage teased multiple possible story beats that could have occurred in a prospective fifth season of the show. This included anti-mutant hysteria resulting in increased Sentinel production, the Brotherhood finding a place with SHIELD as the show's version of Freedom Force and numerous mutants joining the X-Men -- including a reformed Magneto. The most enticing of these proposed storylines would have been the show's adaptation of the Phoenix Saga. While Season 5 wasn't fully written before the show's cancelation, an interview between The Dork Review and Storyboard Artist Steve Gordon revealed Jean's ascent into the Phoenix would have indeed been the proposed fifth season's plot line -- and may have been one of the best adaptations of the story to come in other media.
As one of the most iconic X-Men stories of all time, Jean's transformation into the all-powerful Phoenix and her eventual corruption into the Dark Phoenix has served as a major storyline in two different live-action films and served as the basis for multiple storylines for other animated shows like X-Men '92 and Wolverine and the X-Men. The problem inherent to any adaptation of the Phoenix Saga though is the fact that for the storyline to really work, fans need to genuinely care about Jean Grey's arc and her relationship to the rest of the X-Men. It's the loss of Jean's restraint and emotional bonds that makes her loss so tragic, but Jean was largely a cipher in the live-action films, with both X-Men: The Last Stand and X-Men: Dark Phoenix failing to make Jean's arc compelling -- all the while treating most of the of the cast as afterthoughts.
X-Men '92 adapted many elements of the original story but was as a result left overstuffed and incapable of fully exploring her arc. Meanwhile, Wolverine and the X-Men more or less used Jean and her evolution into the Phoenix as a plot device instead of focusing on her as her own character. However, X-Men Evolution -- a series that thrived on exploring ever-evolving character dynamics between the cast -- would have been perfectly positioned to keep the spotlight on Jean during a proposed Phoenix Saga. The show had teased her immense power and her quietly fierce temper, both elements that would have likely come to the forefront of the series -- and could have played on long-simmering storylines to enhance the plot.
Jean's relationship with the show's version of Scott Summers -- teased for multiple seasons before finally coming to fruition in Season 3 -- could have also served as a major backdrop of the plot, especially if the show finally paid off their long-simmering tease of a possible Cyclops/Rogue romance. This could have kept the conflict around the Phoenix grounded by the main characters and their dynamics, making the actual tragedy of losing one of their own to her own immense power all the sadder. X-Men Evolution spent so much time setting up its very defined and expansive cast in a way the other adaptations didn't have the time or focus to pull off -- and as a result, any Phoenix Saga they attempted would have carried more intimate weight for the audience.