Barry Crain is a storyboard artist on the Legion animated show. Back in September when the show first started airing, he wrote that "Kids' WB/CW is threatening to reintroduce Supergirl in the Legion, should the show be a success. Fingers crossed..." At the time he provided some concept sketches of what she might look like if she did appear. Obviously, that never happened in Season 1, but who knows, she might be the "surprising, new ally from across the space-time continuum" promised for Season 2.
He recently said he doesn't know if WB's comments still hold true, but in the spirit of concept sketches, here's one.
Visit Barry's blog for more concept sketches.