Warning! The following contains spoilers for the Season 2 episode of Legion titled "Chapter 10." Read at your own risk!
Legion's basic plot for Season 2 has been laid out and while fans are aware that David is helping the Shadow King find his physical body at Future Syd's behest, they have no idea why. David didn't seem to know either, so he traveled to the future with Cary and Kerry's help to get some answers. David's conversation with Future Syd helped convince the telepath he needs to stay the course, and also showed viewers some details that might be important later on in this season's story. Here's some stuff fans might've been overlooked during that pivotal scene:
Future Syd Was Missing An Arm And Wasn't Wearing A Glove
David noticed Syd is different in the future but failed to point out the fact that she is missing one of her arms. Weirdly enough, the other arm is not protected by a glove, which could mean a couple things. Legion has shown in Season 2 that Syd has gotten more adept at controlling her powers, so perhaps she's honed her abilities beyond the need to cover her hand in the future timeline. Syd getting comfortable enough with her powers or losing an arm will definitely mean the future David is trying to prevent is rapidly approaching, so fans need to be on high alert whenever she's in danger.
David Couldn't Read Future Syd's Mind
David couldn't read Future Syd's mind and the effort caused his head to hurt. Legion explained this away by the two characters acknowledging it was impossible for David to do because she was from the future (thus filling many plot holes) although that doesn't make a lot of sense. Usually, when a telepath mutant can't read someone's mind it's either because there is nothing to read, or something/someone is blocking them out. If someone else is posing as Syd, it could be a stronger telepath than David. Either that or the future Syd has her mind safeguarded from David, which segues beautifully into the next detail.
David and Future Syd's Relationship Is Complicated
Future Syd appeared to indicate David is alive in the future, but that things between them are "complicated." She also mentioned the reason the Shadow King needs to live is so he can aid in the fight against a threat they'll need to stop when the time comes. Whoever this threat is, they're not around now, as even David isn't more powerful than the Shadow King. Of course, we learned from future Syd that's not necessarily true, as she told David he killed the Shadow King in her timeline a week from their encounter. Legion fans have seen David unchained and the chaos he can create; is it possible future Syd is encouraging David to help the Shadow King in order to give the world an extra ally when her boyfriend goes rogue? If so, that's messed up.