Big Mom being amnesiac could lead to both entertaining or very underwhelming scenarios. I hope she won't be an ally against Kaido, it would completely undermine the significance of the alliance's victory if a powerful pirate from the same generation of Kaido, like Big Mom, would interfere in a way to be crucial for the outcome. This victory should be achieved by the alliance that was formed at Zou, even if Luffy won't fight Kaido 1 on 1 that will still be a victory by the alliance the he formed, and thus a victory as a pirate against the pirate Kaido.
Rather, speculating about what Oda has in store for Big Mom now, we know that the marines intercepted her talk with Kaido and we also know that they aren't too happy about two emperors meeting, especially emperors as hot blooded as them. Akainu was uncertain about sending marine forces at Wano since they don't know the extent of Wano's forces, but if they go there with an admiral (Kizaru, since the other two are at Marijoa) and other strong vice-admirals, and if once there they decide to target the Straw Hats specifically, then Big Mom's role in all of this could be just to stop the marines.
Big Mom was always essentially a child, now that she lost her memories her new personality could vary depending on the people that she meets, and with someone as good natured as Chopper i don't think it's far fetched that she could become good natured as well. Even if the rest of her crew once reunited with her would be furious about supporting the Straw Hats they would never disobey her or abandon her in a fight against an army of powerful marines, thus we would have Big Mom pirates vs Marines in one side and the Pirate alliance vs Kaido and Wano's forces in the other, happening at the same time.
If that will happen i would still think that a memory loss is a lackluster move by Oda, but surely much better that Big Mom helping against Kaido, and i would be really hyped if a double war of such scale should happen.