[Daikaijuman54]

Since day 1, it's been Blades. He has a Seiken SwordDriver, his element is opposite to Saber's, he was the first SOL Rider introduced into the show, he's apparently getting another powerup, yep, sounds like a secondary Rider to me.

As much as Saikou has been filling in that slot, as of now, I don't see any of the other members getting any noteworthy powerups, outside of maybe Espada, should Kento likely return. Though that doesn't exclude the possibility, of the getting new forms come the possible V-Cinemas, or last minute upgrades like Grease Blizzard.