I think Big Mom’s amnesia has been a point Oda has been aiming for since the very beginning and so far he has set this up magnificently. Here’s how.
1-Big Mom already has bursts of amnesia from time to time like the Caramel incident and her other food rampages.
2-Big Mom has been already established to have poor mental fortitude as with the food rampages and her powers literally shutting down over just a picture breaking. So it’s not out of left field that even the trauma of drowning could lead to another psychotic break. This time in the form of prolonged amnesia.
3-Chopper knows Big Mom has an illness and it’s one that Chopper has seen symptoms of in the Punk Hazard children. Oda specifically put Chopper in this situation of finding out she’s sick and then finding her on Kuri.
4-There’s 6 days to the Fire Festival and the last time Big Mom ever went days without food as far as we know was the fast on Elbaf where she went up to 6 days.
5-Wano is in a state of constant famine and Big Mom is here with no food or help from her children.
6-Big Mom has a child who has been specifically demonstrated to have the ability to return people’s memories and yet these abilities have barely served any purpose in the story so far.
7-Big Mom has leverage over Kaido because of something in their past and Kaido has been confirmed to be distraught about this fact. Now Big Mom doesn’t remember anything and I can easily see a plot developing that Kaido wants to keep it that way.
Point is, I can’t say much for how this plot line will progress or get concluded but as of now, it’s been developed and set up perfectly.
*Theory by ImmaIvanoM
