Live Wire , Red Tornado, and Metallo are interesting enough , I'd like to see them return for an episode. Silver Banshee could be recast if needed. She needs a strong and frightening nemesis, the "Reverse Flash" of the Supergirl world. Not sure who that would be.
It looks like Cadmus and Hank Henshaw (CYBORG SUPERMAN CYBORG SUPERMAN) appear to be this season's returning villains.
Given Supergirl's history in comics with the LoSH, I wish they'd have an arc where she would journey to the 25th (?) Century along with Mon-El, help the Legion with whatever, meet Brainiac 5 and get smitten, and then return. Then every now and then Legion enemies could appear from time travel to challenge Supergirl. I'm dreamin', I know.