In my opinion, the main trio works well as is, but after re-watching the series recently (because I had nothing to do... 😅), I wondered what a hypothetical fourth member for the trio might be like. I imagine a character who is a sort of rival for Marcy (like Sasha was for Anne in the first season): almost as smart as her, but unlike her, sarcastic, arrogant and vain. He describes himself as the smartest person in school, and he despises and makes fun of others (especially Anne, who he considers the dumbest person he has ever met), but he has respect for Marcy, who he considers the real smartest student (after him, of course).
But I would also like to know your opinion: in your opinion, what would a new member of the trio be like? What characteristics do you think would integrate better with the group and what dynamics could it create? Or do you think that the Calamity Trio is perfect as it is, without the need for additions?