Depends. I've always said that if Marianne hadn't died, Cornelia would have been more defensively protective instead throwing herself full-force into Operation: One Rule. Her personality would remain the same, as would the extent of her actions — she was severe and capable of ruthless cruelty before Marianne died, imo — but there are different courses that she could have taken. She's always had a knightly attitude, so there's that. She might not have been so quick to refuse the GG position of Japan when Schneizel first offered it to her, so there's that too if you wanted a reason for Lelouch to be in Japan more (she could have taken the vi-sibs & Euphie, since I don't think... the vis, at least, would have much support on the mainland without her, but maybe you want to have them build that themselves, I don't know.)

I was thinking a more "you tell me what you want, I work with it" thing because, like I said, this means so much less for Cornelia than it does for everyone else. It's easiest for me to facilitate.