Based on recent revelations from Chapter 1044, there are multiple theories that can be dragged together regarding the Yonko and Joy Boy.
I believe that all of the Yonko are/were trying to create Joy Boy or replicate what Joy Boy does but all (except 1) had some fatal flaw.
After “defeating” Luffy, he proclaims that he couldn’t be Joy Boy, as if he was in an active search this whole time.
With this panel, we see that Kaido has practically given up on being Joy Boy himself.
Additionally, he seems to be in Wano knowing that Joyboy will appear there one day.
For Kaido, he probably knew that there is some set of conditions to make Joy Boy “appear”.
- 1- The Hito Hito no Mi: Model Nika
I’m not sure about the extent of Kaido’s knowledge, but he does seem to know that there is a fruit that is key to Joy Boy, and that the fruit is a Zoan. This is shown in Kaido’s production of SMILE fruits, all artificial devil fruits which are only zoan.
And of course, the side affect of the fruit is causing people to always be happy, to always experience Joy. These replica fruits may be trying to emulate the Hito Hito no Mi: Model Nika specifically, but it can’t be recreated for some reason, causing both the Gifters and the Pleasures to be just “side effects” of trying to make Nika’s devil fruit.
- 2- Coming close to death, or actually dying
This one I’m not sure on. We know that when Luffy awakened his Nika devil fruit and went Gear 5, he might’ve been dead.
And when voices are disappearing, it has been shown in the past that people are dying:
Kaido, knowing that death was a possible key to becoming Joy Boy, constantly tries to kill himself.
So, Kaido is trying to emulate the summoning of Joy Boy, which ended up happening to Luffy. He sets up all the possible conditions for Joy Boy, established himself in Wano, part of the ancient/great kingdom where Joy Boy once was and ruled over, and still cannot be him. His fatal flaw was that he restricted the Freedom of his people, the opposite of what Joy Boy stood for.
2- Big Mom
Big mom is a different case than Kaido. She seemed to either know less about the “conditions” to bring back Joy Boy, or she went about it in her own way. Still, she attempted to create what Joy Boy managed to create in the ancient/great kingdom: a place for all to live in harmony and joy.
This was taken down by the World Government (formerly the 20 Kingdoms), which forced people living in harmony to separate. I believe that this is why the Fishmen inhabit Fishman Island, and not the surface: to run from the persecution brought after the collapse of the Great Kingdom. Similarly, all of the races in One Piece seem to be very spread out, from Skypieans to Giants. I believe they were previously united under Joy Boy.
Big Mom’s main motive is an emulation of the kingdom Joy Boy built: have people of all races and creeds living together. She so desperately has her children with characters of multiple races, so she can build her own kingdom where everyone can live in harmony.
Of course, she fails to bring the Giants, who she so desperately seeks out.
This reveals that her dream is a selfish one. She only wanted all these people to crush everyone else. Her dream of having all races isn’t to actually unite them, like Joy Boy, but to gain power and bring violence.
It is also important to note that she also makes Whole Cake Island out to be very “joyful” and happy all the time.
This is also a façade, as we know she uses her resident’s lifespans in order to fuel this.
Like Kaido, she attempts to emulate Joy Boy through artificial means, but misses a key point. Her flaw is that she is uniting these people through force, not through their own will, again limiting their freedom. She also attempts to emulate “joy” but is completely unable to, as all her residents live in constant fear of her control and her hunger pangs.