Nightcrawler of the X-Men was originally supposed to debut in the Legion. Dave Cockrum tried to sell him as a member of the Legion. When that did not work out, Cockrum worked on the idea of a spin-off of the Legion that would be called "The Outsiders" and would include Nightcrawler. The editor considered the character too funny looking and rejected him.
When the Sensor Girl concept was first introduced, she was meant to be a way to restore Supergirl after she died in the Crisis. Supposedly her Kryptonian senses got amplified after her resurrection explaining her powers. But due to the infamous embargo on Kryptonian characters in the post-Crisis DCU, the identity was given to Princess Projectra.
Kinetix was going to be the new Emerald Empress in the Reboot Legion, with Jeff Moy having designed an "Emerald Princess" costume for her. However, readers began to suspect this obvious development and instead the Emerald Eye used Shrinking Violet for a host.
Jim Shooter's intended ending for his run on the Threeboot series would've involved Invisible Kid's being revealed as Transgender and implanting her mind in a new body modelled after Gazelle, taking the name "Stealth."
It's been mentioned ever since the end of the last Legion volume that numerous creators kept pitching their own take on the concept, but Dan Didio shot each of them down.
Keith Giffen apparently had the idea of relaunching the Legion continuity as a The Dirty Dozen type of scenario where R.J. Brande was dead, the founders were in Takron-Galtos for his murder, and the Legion was put together from other inmates.
After nearly five years of their status being teased, the Legion was going to be officially reintroduced through Saturn Girl's role in Doomsday Clock. However, due to all the delays by the time Brian Bendis was writing the Superman titles he was allowed to introduce a brand new Legion while Doomsday Clock was still going. Doomsday Clock was forced to rectify this by having the Saturn Girl who'd been running around the Rebirth titles vanish as soon as she admitted Superman didn't remember her. Bendis's Legion appeared in the final issue, ostensibly having replaced whatever Legion would've originally appeared.
Devin Grayson was all for using older versions of the Legionnaires in her Jon Kent story for DC Pride 2022, including the following:
Invisible Kid and Chemical King from the Silver Age.
Invisible Kid and Brainiac 5.1 from the Reboot Legion.
Dawnstar, Lightning Lass and Shrinking Violet, and Element Lad. There would've also been Shvaughn Erin with the Retroboot Legionnaires, due to Shvaughn having been noticeably absent from the Retroboot even after Paul Levitz came back as writer.
The characters would've all been confirmed on-panel as queer (The two Lyles, Condo, and Vi were gay, Ayla and Dawny were bisexual, Brainy would've been identified as demisexual, Jan was pansexual, and Shvaughn would've been confirmed as a trans woman without the messiness from the Five Years Later Legion). But editorial chose to stick with the Legion that knew Jon Kent, so only the Bendisboot versions of Brainy, Dawny, Vi, and Jan were used.
A new Legion adult animated series written by Brian Michael Bendis and developed by HBO Maxwas announced in December, 2021. However, there has been no further information about the project since.
. However, there has been no further information about the project since.