There has been rumblings about the Legion of Superheroes coming to Supergirl since we saw the Legion ring in the Fortress of Solitude in season one. Now you're going to see a lot of people saying that Kevin Smith has just confirmed that they're doing Legion of Superheroes on Supergirl because of the lastest episode of Fatman on Batman. Around the 1 hour 35 minute mark Smith is asked about doing special effects. He uses Supergirl as the example because of how much of the show is left for Special Effects supervisor Armen Kevorkian to do. This is something Smith is fine with now but would've had a problem with when he was younger.
But in it he talks about all the people who are working on the Flash/Supergirl series that come from Smallville and had he done an episode of Smallville then he would've been less worried about doing special effects. He says the Smallville alum aren't phased by anything and talks about how he'd be stunned that the are doing Legion but the Smallville folks wouldn't be because they did Legion back then too. A lot of people on-line are taking that example as confirmation that they're doing Legion. And maybe they are, but I wouldn't consider that 100% confirmation.
Legion would make a lot of sense as they tie into the comic history of Mon-El. After he got lead poisoning and was put in the Phantom Zone to save him, it was Brainiac 5 and the Legion that rescued him. So, we might see the Legion, I just wouldn't take Smith comment as confirmation, especially since he just said a minute before how he didn't want to give away spoilers.
Supergirl returns on October 9th to the CW.