Big Mom is getting defeated in WCI.
Luffy has already said so 3 times in the WCI arc.
Against Katakuri.
Against Big mom in the middle of the arc.
At the wedding against Big Mom.
Also, all villains get defeated in their own territory, proof?
- Arlong got defeated in his territory, Cocoyashi village
- Wapol got defeated in his territory, Drumg Kingdom
- Crocodile got defeated in his territory, Alabasta
- Enel got defected in his territory, Skypiea
- CP9 got defeated in their territory/base, Enies Lobby
- Moriah got defeated in his territory, Thriller Bark
- Caesar got defeated in his territory, Punk Hazard
- Joker got defeated in his territory, Dressrosa
- Kaido is getting defeated in his territory, Wano
Based on trends and evidence, most likely Big Mom gets defeated at WCI which he intends to return to.
- There's also no reason why Big Mom will get defeated in Elbaph.
- She can't go there because she was banished.
- She tried to arrange a political marriage to attain their army to become Pirate King and failed.
She's not a threat to Elbaph, therefore, she can't go to Elbaph. She's even trying to create giants, why is that significant? Big mom takes things by force and if she can't, it means something is stopping her and only force and power stops Big Mom; since she can't take them by force, she's trying to force people to become giants through sicence. There's no reason why Big Mom will go to Elbaph since she can't exactly subdue them.
More so, the places villains get defeated genuinely always need Luffy to save them... Ellbaph doesn't need saving. They're said to have the strongest army in the world. WCI will need to be saved because its being ruled by Big Mom, a tyrant. Luffy's career as a pirate is literally liberating people from tyrants and such.
Yes, they get protected by Big Mom, but she's still a tyrant. She kills. She takes away the freedom of the people of WCI. If they try to leave Tottoland, they will get killed because Big Mom is a dictator who is a megalomaniac and control freak.
While I'm personally 99% confident she's getting defeated at WCI, Oda may still try to switch things up. It's just highly unlikely Big Mom will get defeated outside her territory. There's far too much symbolism that goes with defeating a villain in their own territory, it just makes Luffy seem much more of a threat.
I'm assuming Big Mom will leave Wano before they take on Kaido. Denjiro will probably misinform Orochi who still trusts him and alert the alliance to charge once they've gone. The alliance simply cannot defeat 2 Emperors at once; it's unrealistic at the very least.