"The Time Has Come For New Power!!" Sometime between 1979 and 1982, noted DC heroine writer Jack C. Harris put together a proposal with artist Trevor Von Eeden to promote Supergirl and Batgirl as a genderbent "World's Finest" and create a recurring Defenders-style "non-team" alongside the Enchantress and the Vixen. You'll note that the Lady Fox is still in her original costume from Cancelled Comic Cavalcade #2, so this would have been her second aborted chance at getting published before finally appearing in a comic book story through Action Comics #521.
"Power Plus Two: Inborn ability together with a mystic amulet calling forth the powers of the beasts alter the identity of fashion designer Mari McCabe into the Lady Fix... The Vixen!"DC Women Kicking Ass interviewed Harris about the sadly rejected proposal, which you can read here!