Hmm, I like to think that BigMam's fruit will be the Hunger fruit, not simply stomach acid or acid. I'm not saying it couldn't be either of those, but I like this possibility because her insane hunger would be justified.
I know there's the precedent of Bonney, and that generally speaking strong people in One Piece eat amazing amounts of food and imbibe ridiculous amounts of alcohol, but still, her entire theme is hunger and gluttony, so I believe it would be fitting.
I think that her power bestows her with an insatiable hunger, and her entire metabolism and digestive system are constantly in overdrive. She never stopped eating, and that hunger made her a cannibal, eating the foes she defeats and anything else she found growing ever stronger.
Other powers could be to control the hunger in others, making them weaken or go insane, obviously use her production of super potent stomach acids to destroy things and harm opponents, using her saliva to hinder movement and obviously have a huge appettite and the ability to chew through and eat anything.
Considering Wapol and the fact that it would be very similar there would need to be an extra twist though. But the other option, acid, is also very similar to Magellan both in the type of power and in the fact that there's a side effect to it in the digestive system.
Anyway, I think this would make her the most hideous and despairing of all foes, since her ultimate objective of eating Luffy would be constantly reinforced by the type of abilities she uses. Personally I find that the prospect of being eaten is one of the most horrible and disturbing things, so it would make for a very dark, disgusting and desperate battle, which would go with how Oda has protrayed Big Mam in this chapter very well. Personally, I wouldn't mind this at all.
Anyway, that's my two cents.
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