Throughout WCI it has been hinted pretty heavily that Katakuri wants big mom to be defeated. I mean why were these his last words to luffy? what's more ominous is that the name of the chapter this happened in was titled "the last request", as if this was something katakuri was asking of Luffy. To defeat big mom.
But why does Katakuri want BM defeated? well its pretty obvious. Katakuri's entire motive was to do one thing, protect his family. He supports big mom because big mom protects his family. But its also true that big mom was shown to be an incredibly abusive mother and hence he does not have full loyalty to her imo. This is, partially imo, why he didn't just murder Luffy outright when he had the opportunity and why he was actually cheering on luffy throughout the fight.
Katakuri wanted Luffy to become strong enough to defeat big mom.
And in wano kuni BM actually might be defeated. Luffy would become strong enough to be a yonko. And katakuri could have totland be protected by Luffy instead of Big Mom, freeing his siblings from their abusive mother.
Alternatively, Luffy could have motivated Katakuri to train and Katakuri becomes strong enough on his own to replace big mom, albeit that is a bit far out.
in any case, I think it fully makes sense for Katakuri to want to see the defeat of BM and for him to come to wano, perhaps with other BMP who want BM gone, to help Luffy's alliance.
There are several big reasons why katakuri is needed in wano. The first among them being that
- the alliance needs more men.
- Luffy needs to awaken his DF
By using G4, Luffy cannot use ACoA as effectively, which is a problem. Hence, Luffy needs to learn how to use G4 without haki. The only way to do that would presumably be with DF awakening. The issue is that as of now, there is nobody on the ally's side that can teach Luffy how to use DF awakening.
Now Luffy doesn't need formal training in order to use DF awakening imo. It isn't like ACoA. Luffy has been using his DF since childhood, he already has good mastery over it. All Luffy needs is for a teacher to explain to him what awakening is and how to achieve it, then Luffy can learn the rest. This is where Katakuri could come in, he would help Luffy awaken his powers, allowing him to use ACoA at full potential.
But how would katakuri reach wano?Isn't it a bit to late for him to appear? Well that's where jinbe comes in, imo. Why didn't Oda just have jinbei appear earlier, why is he holding out on him? Imo the reason for this is that Jinbe will play a crucial role in the defeat of kaido by recruiting allies for Luffy. At the end of WCI he was likely captured by BM, along with judge and the vin smokes. While he is in WCI, I believe Jinbe organized a rebellion against BM, getting help from katakuri. This rebellion will soon wash up on the shores of wano kuni to give Luffy the edge against the yonko alliance.