Before the events of the Wano Country arc, the Big Mom Pirates were led by Charlotte Linlin, better known as Big Mom. As the crew of one of the Emperors of the Sea, the Big Mom Pirates were considered to be one of the four most powerful crews in the One Piece world. They were feared by other pirates and the Navy, but with Big Mom losing her position, the crew finds itself without a captain to guide them. This comes at a time when the Navy is taking huge strides to capture the former Shichibukai, so it is possible to assume that they might come after the other famous pirates. Fans have already seen the power of the Navy's newest weapon, the Seraphim.
These monsters have been created by the Special Science Group, which is headed by Dr. Vegapunk. The Seraphim appear to have traits from the Lunarian Tribe, which is arguably the strongest race in the series. Their facial features seem to resemble those of the former Shichibukai when they were younger. A single Seraphim has the power to wipe out an entire pirate crew, and possibly even more, considering that even Blackbeard was taken aback after witnessing their power. There is no information available about the number of Seraphim that the Navy has at its disposal, but even a small number of them can be dangerous to the pirates of the world. As Whole Cake Island is not protected anymore, it is plausible that the Navy might make its move. So, the Big Mom Pirates need to regroup as soon as possible.
The Next Leader
In the past, whenever there was a debate about the potential leader of the crew after Big Mom, two names always popped up: Charlotte Perospero and Charlotte Katakuri. As the eldest child, Charlotte Perospero had a very good chance of becoming the next leader of the crew, but after his defeat in Wano at the hands of Nekomamushi, it seems that he is out of the running.
This leaves Charlotte Katakuri as the lone contender, and needless to say, all of the Big Mom Pirates would be content with him as their leader. He has shown that he cares about his family more than he cares about titles and treasures. With Katakuri at the helm, the Big Mom Pirates would become a force to be reckoned with, and they should be able to repel most Naval invasions, but there are many characters that even Katakuri can't beat, and they might have an interest in the Road Poneglyph that is in the possession of Big Mom.
One such character, who might make his way to Whole Cake Island, is Blackbeard. There are many theories that speculate Blackbeard will go to Whole Cake Island, and a fan even created a few pages that showed the fight between Katakuri and the Blackbeard Pirates.
Obviously, Blackbeard is interested in claiming the legendary treasure and becoming the Pirate King. So, this would be the perfect opportunity for the Emperor to claim the Road Poneglyph and get closer to his goal. Blackbeard might also be able to get his hands on some powerful devil fruits that belong to the Big Mom Pirates. So, what can Katakuri do to keep his family safe?
Swearing Fealty
One thing that every weakened nation can do to avoid being destroyed is to pledge allegiance to someone who is stronger. Fishman Island was under the protection of Whitebeard, Big Mom, and now Luffy, which kept the country safe from any kind of harm. This is a very effective and safe method that can potentially decide the fate of an entire nation. So, who can Katakuri join to prevent the destruction of Whole Cake Island? The answer is the Straw Hat Pirates.
Toward the end of the Dressrosa arc, the Straw Hat Grand Fleet was formed, and it included many famous pirates, such as Cavendish, Bartolomeo, and Don Sai. The Straw Hat Grand Fleet definitely has a role to play in the story, which is why Oda is saving them till the end. If Katakuri were to join the Grand Fleet, it would mean that he would be under Luffy's protection. Clearlt, this is a viable option, but will Katakuri agree to do such a thing?
During his fight against Luffy, Katakuri was very impressed by the young pirate's willpower. The two of them fought until only of them was standing. After Luffy woke up, there was an interaction between the two where Katakuri asked Luffy if he would ever come back to topple Big Mom, and the latter said that he would since he would become the Pirate King. In response to Luffy's answer, Katakuri said he was looking quite far into the future, and then he collapsed on his back.
Before leaving, Luffy left his fedora on Katakuri's mouth because he knew how people treated him in the past. This was a sign of respect between two incredible pirates. This is further backed up by Katakuri and Brulee's exchange, where she asked her older brother why he fell on his back after initially falling on the front. Katakuri did not have a concrete answer, but this insinuates that Katakuri was happy that his immaculate record came to an end after his fight against Luffy. All in all, it makes perfect sense for Katakuri to seek out Luffy instead of risking the lives of his siblings.