I don’t see in what way Black Swan would challenge Carol. It would have to be with other baddies or something given Carol’s treatment of Black Swan and Black Dwarf in the Star book
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Unless they give Karla another Moonstone or have her come at Carol another way i don’t see how she’d challenge Carol. Carol kinda easily handled her the last two times Karla has appeared in a Carol centric comic. Once in the Brian Reed run with Carol regaining both her halves and like 2-3 shotting Karla and taking the Moonstone and then by Thompson herself in the Life Of Captain Marvel book easliy handling Karla. If Moonstone came back she’d need a amp. Especially since Carol can apparently go Binary for short periods of time without any notable energy absorption under Thompson. Karla really isn’t a plotter either. She needs a amp to be a legit threat to current Carol
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The idea of Moonstone and Star interacting sounds like something that should happen though.