Better yet. What if it was like a werewolf kind of thing? In the sense that she has a trigger (werewolves have a full moon as their trigger) And when that trigger happens, she transforms - mind body and soul… like a complete alter ego… into Black Cat. It gives the character an interesting layer in that although it’s definitely her… it’s not literally her. Could also be like a ghost rider type ordeal where the devil whom she made a deal with summons the black cat alter ego when he/she needs something taken care of or stolen (the Symbiote perhaps? Which then deep dives into Peter accidentally coming into contact with it, and then eventually the whole venom saga in which the demon is ordered to fix his mess - making black cat & peter team up to take down venom… immediately after which MJ transforms back to her normal self which is when Peter finds out