Noah Hawley has hinted at the possibility of Charles Xavier appearing in the series. It is currently unknown if Patrick Stewart or James McAvoy would reprise their roles. Although Patrick Stewart has said that he would "absolutely 100%" reprise his role on the show despite previously claiming that "Logan" (2017) would be his last time portraying the character.
The character of Syd Barrett who is played by Rachel Keller is named after Syd Barrett of the rock band Pink Floyd, whose music was an important influence on the series for Noah Hawley. Barrett left Pink Floyd in 1968 due to mental illness issues.
The show is part of the X-Men cinematic universe as it connects to its future films while acting as a standalone series. However, it's hinted that it exists in its own universe and not part of the X-Men universe.
In one of the episodes as Cary Loudermilk (Bill Irwin) meets the man in the diving suit, he greets him in several languages. This is a reference to the book "Twenty Thousand Leagues Under the Sea" by Jules Verne, where Professor Pierre Aronnax also greets Captain Nemo in several languages not knowing what language he can speak.
The character Syd Barrett is frequently shown early in the pilot episode with light reflecting around her head. This is a possible homage to the Pink Floyd tracks "Shine on you crazy diamond" parts 1 and 2 which were about former lead singer Syd Barrett who was also considered mentally ill.
At the beginning of the series, David is diagnosed with schizophrenia. The other main character, Syd, is named after Syd Barrett (of the band Pink Floyd), had to leave his band after he was diagnosed with schizophrenia.
Jean Smart and Rachel Keller both starred in the second season of FX's Fargo. Jean played Floyd Gerhardt who was the grandmother of Rachel's character Simone Gerhardt. Noah Hawley was so happy with their performances that wrote the roles for them.
The trivia items below may give away important plot points.
In Marvel Comics, Legion (David) causes the alternate universe of a world without Charles Xavier. In this world or the "Age of Apocalypse" Xavier was killed and his legacy of starting the X-Men (et al) was changed. Magneto was the Headmaster of Xavier's School of the Gifted, Apocalypse rules the world, some good guys were evil (and vis versa), only one of the "X-Men" knew the alternate timeline was different/changed.
The title of the show is based on David Haller's codename in the X-Men Comics. David acquired the codename "Legion" due to having hundreds of multiple personalities each with his/her own mutant power. Essentially making him a one man army.
In "Episode 7" David describes his fight between his father and "The Monster". His father is drawn as a bald-headed mutant in a business suit and David then makes fun of his father's British accent. This is an allusion to David's father in the comics (who is Charles Xavier) and is played Patrick Stewart with a British accent in the X-Men movies.
In the montage of David growing up, David sets an experiment on fire while at school and later, a convenience store burns behind David as he walks away. In the comics, pyrokenesis is a power that David commonly uses.
Easter Egg: In the Chapter 7 episode, during a flash of memories or future things to be, you can briefly see the "X" wheel on the wheelchair always associated with Charles Xavier (Professor X). This may further the rumor that Xavier may, at some point, have a part in the show. It's unknown who will play the professor if that happens.
David Haller is the son of Charles Xavier in the comics. The character has a severe mental illness in the form of a dissociative identity disorder, which grants him multiple personalities. James McAvoy who plays Xavier in X: First Class (2011) also stars in Split (2016) as a man who suffers from a very similar condition.
In one episode, in a discussion regarding a major location in the series, Aubrey Plaza's character says "I mean, what kind of a name is Summerland?". Oddly enough, there is a town called Summerland in British Columbia, roughly 250 miles north of where the series is filmed.