The Yonko, also known as the Four Emperors, have existed in the world of One Piece for quite a long time. Since Roger started the Great Pirate Era, countless pirates took to the seas to assert their superiority over them and finally, get to the grand treasure that he left behind, called the One Piece. Out of all the pirates, four took the era by storm and started to rule over the seas like kings and they're the pirates who everyone knows as the Yonko.
Currently, the Yonko are made up of Big Mom, the Queen of Tottoland, Kaido, the Supreme Commander of the Beasts Pirates, Shanks, the Chief of the Red Hair Pirates, and Marshall D. Teach, the Admiral of the Blackbeard Pirates. Undoubtedly, these four pirates are the mightiest on the seas right now, however, their age might be heading closer to its end sooner rather than later.
The Fall Of Kaido And Big Mom
One Piece's author, Eiichiro Oda, finally started focusing more on the Yonko after the timeskip. With Luffy heading into the New World, it was clear that the time for him to challenge the supremacy of the Yonko had arrived. On Fishman Island, Luffy picked a war with Big Mom, while Punk Hazard saw him form an alliance with Law to take Kaido down. Moving forward, Luffy even clashed with Big Mom during the Whole Cake Island arc, however, it wasn't until the Wano Country arc that he took his first step towards becoming a pirate capable of rivaling the Yonko.
In Wano, Luffy and the Straw Hat Pirates formed an alliance with the samurai of Wano and even the likes of the Worst Generation to an extent, in order to combat the joint power of Kaido and Big Mom. Without a doubt, this task is humongous yet, together, the Worst Generation seems to be prevailing. After all, Big Mom has already been defeated by Kid and Law, while Luffy is currently still battling Kaido over the Flower Capital. While it is quite clear that Luffy's fight against Kaido won't simply see him win easily, at the end of the day, the land of Wano has to be liberated and the alliance has to prevail. Luffy will inevitably win against Kaido and when that happens, two of the Four Emperors in the series will have fallen.
This is, quite clearly, unprecedented in the One Piece world. The fall of two Yonko means that the entire balance of power will be shaken and the system of the Yonko could eventually come to an end, to be replaced by a better system with more powerful people in it.
The New Era After The Yonko
It must be noted that the fall of Big Mom and Kaido certainly doesn't mean that the Yonko don't exist anymore. Shanks and Blackbeard, two of the most important characters in the story are still alive and breathing, after all. However, just two Emperors leave room for two more people to climb the stage, which is where things get interesting. Luffy is certainly one who fans could argue will take one spot.
After all, he is already the Fifth Emperor of the Sea. However, the last spot has quite fierce competition in Kid and Law, both of which did equally good against Big Mom. Given how Oda has been portraying them, it is highly unlikely for him to just give the spot to one of them and not to the other. As such, fans could see both rise to a position of power.
Instead of the Four Emperors, fans could potentially see a completely new system with five powerful pirates in them instead of just four. While arguably this could be the new system after Wano, there is another interesting group that is yet to be resolved and that is the former Shichibukai. After their dissolution, these pirates are now enemies to the World Government, meaning they're no different from any other pirate and have lost their special status. That, however, certainly doesn't mean that they've lost their power as well.
The Shichibukai were chosen to oppose pirates because of their mighty strength, and with the fall of two of the Yonko, they could potentially be thrown into the mix as well. Buggy, especially, is one character who could be thrown together with the groups given how he always makes his way to the top despite actually being very weak. Meanwhile, Boa Hancock is extremely powerful and Mihawk is a former rival to Shanks, who is currently a Yonko. Edward Weevil is, by the looks of it, a monstrous pirate whose strength is close to that of a younger version of Whitebeard, meaning he's immensely powerful. Given that there are many powerful pirates left on the seas, it is highly likely that a completely new system will be invented which accommodates the two remaining Yonko, the Worst Generation, and the Shichibukai, together as the strongest pirate force on the seas.