Legion doesn't immediately announce itself as an X-Men series. In fact, some of the only things that carried over from the comics to the show were the general idea of the main character (in a much different form than his comic book self), and the use of the word mutants to describe super-powered humans. The inclusion of a new character from the comics could signal the upcoming arrival of a major X-Men character, David Haller's father. The Shadow King, aka Amahl Farouk, hinted at the power of David Haller's real father, but could Professor X be making an appearance on Legion?
After revealing herself to David as The Devil With Yellow Eyes, The Angriest Boy In The World, and his dog King, among other visions that have plagued him his whole life, Lenny started talking to David about his father — his real father. In the X-Men comics, David Haller is the son of none other than the founder, leader, and namesake of the X-Men, Charles Xavier aka Professor X. With Legion drawing a clear parallel to one of the most recognizable mutants in the entire franchise, the Shadow King's relationship with Professor X could be the bridge that connects Legion to the wider world of X-Men.
The Shadow King, in the form of Lenny, taunted David with the knowledge with this father. "Talk about an a***ole. Always acting so holy and gives away his only son. He thought he could hide you from me, but he was wrong. I found you," the character said to David. Professor X is often portrayed as a good and honest figure, a hero among heroes. His alleged abandonment of David casts a shadow over the otherwise pristine reputation of Professor X — but The Shadow King is far too manipulative to take anything said at face value.
While the details of Legion's version of Professor X is still shrouded in mystery, the show is readily aware that The Shadow King and Professor X have history — even more than The Shadow King has with David. The Shadow King has long been a thorn in the side of Professor X, inhabiting different forms and taking over different mutants in an effort to attain power. No matter how he's depicted, it's clear that The Shadow King is in search of power, and it's Professor X's goal to make sure that they do not get that power. David may be able to handle The Shadow King on his own, but the nod to Professor X means that Legion could start expanding into the established X-Men universe, or establishing its own.
Before the series premiered, producer Lauren Shuler Donner spoke to IGN and revealed that "we're making [Legion] separate. We're our own world [unconnected to the X-Men film franchise]." Because Legion and the X-Men films are separate entities, either of the Professor Xs that fans are familiar with from the films may not be making an appearance, although star Dan Stevens has spoken to Patrick Stewart about appearing on Legion, which Stewart seemed down with. While Stewart making an appearance would be a welcome surprise, a new actor could also take up the mantle of Professor X to give David Haller some much needed fatherly support. With the threat of The Shadow King looming over everyone, this ever-powerful Professor may need to show up sooner rather than later.