I try not to make theories on anything I read unless I am positively sure that what I have to say has some weight. So, although you might disagree, please don't think I just pulled this out of my ass.
I first came up with this theory when I read a comment on the sub. "I sure hope that Oda finds a way to keep Law involved after this arc ends." I don't remember who said this (probably a lot of people, to be fair) but it did get me thinking...how would Law stay involved after this arc is over? He won't join the Grand Fleet, Law is too proud to bow to Luffy. So...how?
And then I started thinking about something else...the power vacuum. Let's be real, the only two powers in the One Piece world are the World Government and the Yonkou. The Shichibukai mean nothing in the grand scheme. Other than possibly Mihawk, none of the current Shichibukai can actually compare to a Yonkou's strength. So, on this basis that the World Government and the Yonkou are the only two BIG powers in the world...what happens when Luffy beats Kaidou and Linlin? Simple answer: a massive power vacuum. The government, undeterred by the threat of two of the biggest Yonkou could swoop into the New World and gain a massive power advantage. Losing Linlin and Kaidou would, almost by default, ensure that the Government would end the Great Age of Piracy...unless something were to replace them.
Now, I've given this a lot of thought and I'm making a couple leaps here, but hear me out. I'll state it simply, I think that Monkey D. Luffy, Trafalgar Law, Eustass Kidd, Charlotte Katakuri, and either King or Queen from the Beast Pirates are going to form a super group and fully supplant the Yonkou as the dominant power in the New World. Why do I think this? I think Oda foreshadowed it very recently. When Garp mentioned the Rocks Pirates, he was dual foreshadowing. First, the Rocks will definitely be an antagonist in the future. And given that I think Big Mom and Kaido were both part of the Rocks, they might already be antagonists in the story. Second, the Rocks showed that there was a power before both Roger and the Yonkou that ran things in the New World. So, if the concept of the Yonkou is a recent thing, then that gives way for them to be supplanted by a different power as well. Now, let’s get into why I think Luffy, Law, Kidd, Katakuri, and King/Queen will be the power that replaces them. And why these five will inevitably be the ones that fight over the title “King of the Pirates”.
First, the easy ones. Luffy and Law are both established as main characters of the story of One Piece. Luffy is THE main character, and Law has gotten just as much backstory and character building as any member of the crew. People on here are right when they say that Oda has to do something with Law, he's built so much investment in him that he can't just be done when this arc is over. Simply put, the Straw Hat Pirates have always been the main characters of the show and Law (and by extension his crew) have become main characters since near the beginning of the time skip. Their places are assured. Now for the three that require explaining.
First let’s talk about Kidd. Poor bastard has gotten the shit end of the deal since his post-timeskip reveal. He’s attacked and failed to kill Big Mom and his plan to try and kill Shanks was literally body slammed into the ground by Kaidou. Now he’s in Beast Jail. But, that’s why I think he’s so fit for this role. Apoo and Hawkins might have done the “smart” thing and joined Kaidou, but they also proved that they don’t have the balls to make any power moves in the New World by doing so. Kidd would rather have his ass beat and say he stuck to his guns than lower his head in deference to someone. He is, in essence, a darker Luffy with less of a moral compass. I don’t think it’s too far of a reach to say that Luffy is going eventually spring Kidd from prison, we learn Kidd’s probably tragic backstory, and then he teams up with Law and Luffy. Then, by the end of this, Kidd will end up on the same level as Law in terms of importance to the story. In a sense, Oda has even foreshadowed this with the three of them fighting the Marines on Sabaondy. Go and look at that panel and tell me that Oda isn’t just screaming in your face “THESE THREE ARE SUPER IMPORTANT!”
Now, Katakuri. People will say I’m being a Katakuri fanboy here and...yeah. I am a Katakuri fanboy. But Oda kind of made it hard not to be. Katakuri is the one halfway decent person in the sea of assholes that is Totoland. He and Luffy have a personal respect for one another and Katakuri even indirectly admitted that Luffy has a better chance than his own Mother at becoming Pirate King. Plus...look at that smile at the end of the Totoland arc. Katakuri is happy that Luffy is still alive. They aren’t Nakama, but the two of them don’t hate each other by any stretch. Furthermore, when Linlin is defeated it’s not as if the Charlotte Family will disappear. Someone has to lead them when she’s gone and, despite his status as oldest, Perospero is nowhere near imposing enough to be the man in charge. Katakuri is imposing, well-respected by his siblings, and the next strongest person on the in the family behind Linlin herself. He will lead them when she’s gone. Now, if I may be just a bit speculative, here are my thoughts on why he will side with Luffy and why it will most likely be before Linlin is defeated. I don’t think Katakuri is happy with his Mother’s leadership. That man shows too much love and care for his siblings to simply take her “murdering” Opera and Moscato on the chin. In a sense, he almost seemed relieved to hear that Luffy would be coming back to beat Linlin. And, I think when push comes to shove, Katakuri would side with Luffy to take her down. Because he, just like Lola and Chiffon and Pudding and Praline and who knows how many others of his siblings has realized that there is a better life to be had without Linlin in charge.
Now, this is the biggest leap. One of Kaidou’s subordinates...either King or Queen...is going to jump on this bandwagon as well. I don’t know who, and I can’t place why yet...but I am certain it will happen. We have seen the precedent of Luffy pulling all sorts of people onto his side before. He’s forced people, through sheer determination, to believe in him. Law sees him as an ally, maybe even a friend. Kidd respects him enough to be excited he’s alive. And Katakuri sees him as a worthy opponent. I don’t think it’s too much of a stretch to think that one of Kaidou’s commanders will eventually be swayed as well. King and Queen are wildcards and I can’t say why they’d do it, but trends in this story that we’ve seen so far makes it uber likely that one of them is dissatisfied with Kaidou enough that they would turn on. And besides, they can’t all be a stupid and bloodthirsty as Jack, right? Cuz if every member of Kaidou’s crew is as much of a dumbfuck as Jack is...I don’t think they would be able to exist as they currently do.
Now I’m just going to go into wild fanfiction mode, but here’s how I think this group will come together. Luffy and Law are on Wano. They free Kidd and he joins them. These three beat the Calamities (Luffy gets King and Kidd and Law take down Queen and Jack). Kaidou disparages his Calamities and either King or Queen, scorned by their Boss, makes the leap to join Luffy’s side. Kaidou, having lost a Calamity and suitably enraged, agrees to accept Linlin’s help to murder these little upstart pieces of shit off of the face of the Earth. Linlin is happy to share, hating Luffy and Kidd for launching attacks on her home, as well as hating Kidd for sinking several of her ships. Luffy (and the Straw Hats), Law (and the Heart Pirates), Kidd (and the Kidd Pirates), and Calamity (and their ship of followers) regroup and prepare for a war. Linlin joins Kaidou and brings the full force of her Family. She orders a pre-emptive strike, sending her “lowly failure” of a second son to go and try to regain his honor. Katakuri goes and takes Daifuku, Oven, and Brulee with him. In secret, he also takes the imprisoned Vinsmokes and Fishmen Pirates with him. Pudding also tags along as a stowaway. Katakuri arrives and, rather than fighting, offers to join up with the Straw Hat alliance. Daifuku and Oven don’t question their beloved oldest triplet and Brulee, although scared, loves Katakuri more than Linlin. Pudding joins as well out of love for Sanji but seems to be hiding something. This alliance goes to war with the Big Mom/Kaidou alliance and are joined mid-fight by the Straw Hat Grand Fleet (minus the Happo Navy and Tontatta Corps). Eventually, the combined might of Luffy, Law, Kidd, Katakuri, and the Calamity topples Big Mom and Kaido. And they celebrate their victory with a wedding. Pudding, of course, re-proposing to Sanji with the memory she took from him of their kiss.
This news makes it to the World Government on the last day of Reverie! Straw Hat Luffy has successfully lead a group of infamous names in toppling two of the biggest pirate threats on the seas. Collectively, Big News Morgans has declared that these five, in their victory, are closest to attaining the title of “Pirate King”! Sai and Leo are both bummed that they missed the Grand Fleet’s first fight, but it’s good that they’re there. King Cobra has been murdered, Shirahoshi has gone missing, and someone has to spirit Vivi, who is being targetted by Rob Lucci, away to the Straw Hats. Thus setting up the next arc involving Pluton, Poseidon, and the missing Uranus...
Sorry, I got a little too fanfiction-y at the end there. But, I do think that most of this is likely to happen. It would give Luffy a way to beat Kaidou and Linlin without having to pull a super power out of his ass as well as give a reason for several popular faces to maintain a presence in the story. Luffy’s title as an “Emperor” is cemented in him getting credit as the “”””MASTERMIND”””” behind all of this and the other four are made out to be massive threats as well.
It elevates the story without pulling a Dragon Ball Z. Luffy doesn’t need to unlock Ultra Instinct when his “most dangerous power” is working overtime to help him win. He beats Kaidou and Linlin, two of the most selfish people on the planet, with the power of friendship.
And, it gives Luffy something he’s desperately missing as a character. Rivals. We all know Luffy is going to be King of the Pirates at the end, but his lack of steady competition has always bothered me. He doesn’t have fellow contenders to the title, he has roadblocks to smash. I feel that giving him Law, Kidd, Katakuri, and this Calamity (all prideful characters in their own right) to race against for the title of Pirate King would make the already interesting story of One Piece so much more interesting!
And that doesn’t even get into what these four would add to Luffy’s Final War against the Government when Dragon inevitably fails and Luffy gets another chance to save a brother from execution.
But, this has been long (three pages in Word). So...I thank you for reading and I hope to hear your own thoughts in the comments. :3
TLDR: Luffy, Law, Kidd, Katakuri, and a Calamity are gonna beat Kaidou and Big Mom’s asses and then race to become Pirate King.