When the trailer for Fox and Marvel's Legion premiered at Comic-Con 2016, a lot of fans were excited about how the series was shaping up. The show pays homage to its roots, set in a world where mutants exist, though from what we've seen in the trailer there isn't an X-Man in sight. This isn't exactly a surprise, as there have been a number of comments made by the producers regarding the fact that this show is separate from other Fox properties such as the X-Men movie franchise.
While the show could have its own X-Men team appear at some point, those involved with the show say that fans shouldn't expect them anytime soon. Some feel that it's a missed opportunity to not have Legion connected to the X-Men, and to the larger cinematic continuity of the X-Men films as well. Would that make it a stronger show, though, or is Legion just fine without being set in the established X-universe?
The History of Legion
Legion is a series about a man named David Haller, one of the most powerful mutants in existence. In the comics, he was also the son of X-Men founder Charles Xavier (though it's unclear if his parentage will be explored within the series). His power manifests as different personalities within his mind, with each personality having a different power. In many of his appearances in the comics, David was trying to gain control over the various personalities who were fighting for control of his body. He's been involved with various X-Men teams in the past, though his problems controlling his powers have also left him comatose or unstable to the point of being sedated at various points in his history.
Though David is generally a good man, his fractured mental state and difficulty controlling his powers have caused problems in the past. He served as a host for the psychic entity known as the Shadow King at one point, and accidentally killed his father (setting off the Age of Apocalypse story arc) when he travelled back in time with a plan to assassinate Magneto. In 2012's relaunch of X-Men: Legacy as part of the Marvel NOW! initiative, Legion also discovered that he was prophesied to destroy the world when he ultimately loses control of his powers. This leads to a number of personal revelations, as well as some major sacrifices as he tries to find a way to avoid that destiny and somehow save the world.
The Standalone Legion
In developing Legion, the goal was to create a standalone series that didn't rely on other mutant characters to carry the plot. Showrunner Noah Hawley explained early on that the iconic characters from the movies all "come with rules" that could affect the direction of the series. There may be an exception made for Professor X (though likely not the James McAvoy version), but even that seemed up for debate at one time. It's obvious from the trailer that other mutants exist, but they may not be mutants that most viewers have heard of. Even if there are some established mutant characters in Legion, they may be lesser-known mutants who we aren't likely to see in upcoming X-Men movies or other live-action properties.
This isn't necessarily a bad thing, however, especially in the show's first season. Given how strange David Haller's character can be at times, he doesn't need to be trapping X-Men in his mind or getting into a fistfight with Cyclops right off the bat. If the show proves popular then it's possible that Hawley and the producers could introduce more famous characters down the line, though they still will most likely remain separate from the X-Men franchise characters.
Even if major X-Men characters show up in the series at some point, the separation between Legion and the big-screen X-Men franchise would mean that different actors would portray those characters. This could help to keep costs down (because good luck booking Michael Fassbender for a TV arc on the cheap) and open up storytelling options, since the show won't be beholden to what's appeared in the films plot-wise. It could also open the door for Legion to appear in a future X-Men film (though likely portrayed by someone other than Dan Stevens).
A Missed Opportunity?
When Agents of S.H.I.E.L.D. premiered on ABC, fans were excited at the prospect of the Marvel Cinematic Universe spanning movies and television with a single universe full of shared continuity. The "connections" in that universe have been pretty lackluster thus far, with the movies largely ignoring Marvel's TV shows, but the overall idea of the shared universe isn't a bad one. Ignoring the possibility of rights issues behind the scenes, could Fox have gone the shared universe route with Legion and the X-Men franchise?
If Legion were part of a shared universe with the X-Men franchise, it likely would have been in name only; there's a chance that certain stars could have made cameos in Legion at some point (similar to Samuel L. Jackson and a few other Marvel movie stars popping up on Agents of S.H.I.E.L.D.), but more than likely the biggest connections would simply be mentions of other mutants or events. Even cameos could be problematic, since the last few X-Men films have taken place decades in the past while Legion seems to take place more or less in the present. Unless the X-Men recruited a new mutant who stopped the aging process for those around him, having X-Men characters pop up looking like they did in the '80s might seem a little off. Having people frequently referring to events that happened in the '70s and '80s might be weirder still (though there are a few places where references could be worked in).
Legion Works Best on His Own
Shared universes can be fun, even if they can be a bit hit or miss with their implementation. In this case, though, it's likely for the best that Legion is its own beast and not part of the existing X-Men franchise. Not only does that give the producers more time to focus on David Haller and his struggle to contain his powers, but it also avoids a forced or awkward feeling that might come when movies taking place decades in the past need to be referenced in the present. If Legion needs connections to the X-Men or more famous characters, it can include its own version of them and avoid being shackled to the existing franchise's plot.
Despite Legion being involved with various X-groups over the years, he honestly works best when he's on his own. The Legion-centric run of X-Men: Legacy was a great read and really explored the character, and the X-Men who appeared in it were largely secondary to David himself. The only one in the run who would make a great addition to Legion was Blindfold, and given how unlikely she is to headline X-Men movies anytime soon it's entirely possible that she could show up at some point anyway.
Legion is expected to appear in early 2017 on FX.