I watched Legion again recently and have been wondering what Lana's special destiny was supposed to be that Imra (Saturn Girl) mentioned to her in their discussion. Surely it wasn't to become Kryptonite Woman and forever be toxic to Clark?
Ahh, but that's just a small part of what happened to Lana. Remember, she's still superhuman, and she's dedicated to doing good. Sure, she can't be around Clark, but her whole life and identity shouldn't be defined merely as an extension of a man. Indeed, being forced to stay apart from Clark frees her up to become her own independent superwoman, and thus potentially gives her a greater destiny than she would've had being just Kal-El's Gal Pal. So it all fits together perfectly. Her destiny is to establish a heroic identity of her own that has nothing to do with the Last Son of Krypton and is thus not overshadowed by him.
Of course, what they really meant was that they were writing Lana out of the show for good but wanted to give her character a positive resolution. So they compensated for "You'll never be seen on the show again" by adding "But you're going on to a great destiny offscreen."