I don't see Big Mom being an antagonist in another arc. Of all the awful things Oda has done to her, that would simply be the cruelest. She has to go down in Wano, otherwise the would be no point in her even being there.
Of all the possible fates though, getting beat up by Kidd & Law, getting her head caved in so she becomes O-Lin, possibly drowning for the umpteenth time....I can't see any of them being good for her. Leaving to only get beat up by Luffy after no one considers her much of a threat anymore would just be the worst though.
She'd be better off just becoming a permanent racetrack for Franky to drive his bike over to perform stunts on.