Right now Kaido and Big Mom plan to find the Ancient Weapons and the One Piece.
So Kaido and Big Mom are aiming for an Ancient Weapon. We have 3 possibilities:
1- They will go for Poseidon. It is possible because Caribou was present when Poseidon was revealed and Caribou was Kaido’s prisoner as well, definitively he is the kind of coward that would give this kind of information to save his life.
Pros: Big Mom threatened to destroy Fish-Man Island in the past over some candies and now the Sun Pirates betrayed her and escaped. So she has personal reasons to want to destroy the Fish-Man Island.
Cons: Caribou was in jail, and for that kind of information he could have gotten a better deal.
2- They are aiming for Pluton. Most likely Pluton is in Alabasta. How would Kaido know this information we can only speculate? Maybe he already knew (not so probable). Maybe for the events in the Reverie that are somehow related to Alabasta.
Pros: After reading the newspaper Blackbeard tell to his nakama that they will sail to take “something” before the Marines. Big Mom and Kaido suddenly switch from being enemies to allies, as if something relevant happened for them to forget their rivality. And this change happens in the same days as the Reverie conflict.
Cons: Kaido’s is going to the Flower Capital so we don’t see any urgency in leaving Wano. It’s stated that for the world the most important news about the Reverie was the Shichibukai abolition but something about the Ancient Weapons sounds more relevant to me.
3- They will go for Uranus. We literately don’t have any details of Uranus. So I can only list some wild ideas:
- Uranus is in Wano and Kaido knows
Pros: Wano history is deeply connected to the Void Century. Kaido established his base there over 20 years ago. Momonosuke presented some kind of unique power when communicating with Zunesha and only Luffy was able to hear it. The only parallel we have is when Luffy hears the Sea Kings, the same ones Shirahoshi was able to communicate with.
Cons: Why does Kaido need the Big Mom Pirates alliance if he have ruled over Wano for more than 20 years?
- Uranus is in God Valley, hidden by the World Government
Pros: Sengoku says that Rocks D. Xebec wanted to conquer the world and was involved in the tabus of the world. Somehow attacking God Valley was relevant for that goal. And of course, Kaido and Big Mom were there so they might know something. Shiki confronts Roger and tells him that he knows the location of an Ancient Weapon. Roger and Shiki took part in the God Valley Incident as well. The World Government completely removed all information of that island.
Cons: For me, Blackbeard is portrayed as the Rocks D. Xebec of this Era, so I think what is in God Valley is the last Road Poneglyph. If the World Government have an Ancient Weapon in their power, they most likely would use it to conquer the seas, unless its use require some special condition.
- Uranus is in Elbalf and Big Mom knows
Pros: Big Mom was very confident that if her alliance with the giants had happened she would have already conquered the world. But a giant army doesn’t seem to me enough for that, there must be something else. The giants with an Ancient Weapon would be so strong that not even the Yonkos can take them, but for 2 Yonkos might be possible.
Cons: Pure speculation as we don’t know a thing about Elbalf.