I doubt Big Mom will end up being a true enemy to the Straw Hats like Kaidou is being set up as. The entire seducing woods section, as well as her tone when talking to Luffy during the Fishman Island Arc, seem to suggest she's more interested in testing Luffy as a potential future subordinate rather than facing him as an enemy, since it's been made clear several times her crew weren't going in for the kill immediately. I feel that Lola's Vivre Card will play a major role in the conclusion of the arc, likely causing Big Mom to ally with the Straw Hats. Alternatively, she could be killed off by the Vinsmokes by being drugged, possibly with the help of Caesar's gas to weaken her crew as well, making the Vinsmokes the staple villains of the arc.