The Yonko are the four strongest pirates in the world of One Piece. Along with the Marines, and the Shichibukai (who have now been replaced by the SSG), they make up for the 3 great superpowers in the series.
Said to be the 'Kings of the New World', the Yonko rule the seas with absolute authority, and even the World Government is reluctant to get in their way. Although there are already four Yonko in the One Piece world, the current ones aren't irreplaceable. New characters can certainly rise to this position in the future, and we've named 5 who have that potential, and 5 who don't.
10 Can Become: Monkey D. Luffy
The Captain of the Strawhat Pirates, Monkey D. Luffy is inching closer to the top every day. Having already been declared as the Fifth Yonko of the Sea, Luffy is closer to the spot than anyone else. He has acquired a bounty of 1.5 billion berries, and, right now, he's at Wano Country with the aim of taking down Kaido, one of the Four Emperors of the Sea. Luffy might not be at their level yet, but he's surely getting closer, and once he's dealt with Kaido, he might officially get to be a Yonko.
9 Can't Become: Basil Hawkins
Basil Hawkins is the Captain of the Hawkins Pirates, and, just like Luffy, he is currently at Wano Country as well. As one of the members of the Worst Generation, Basil Hawkins is a deadly pirate. He holds the power of the Paramecia type Wara Wara no Mi which allows him to manipulate straw according to his will.
Hawkins has earned a bounty of 320 million berries over the years, which suggests that he's not a rookie anymore. However, Hawkins is certainly not capable of becoming a Yonko. He's already submitted to Kaido once, and, after being defeated by Trafalgar Law, whether he is alive or dead remains unknown as well.
8 Can Become: Eustass Kid
Another member of the Worst Generation in One Piece, Eustass Kid is perhaps the cruelest of its members. With a bounty of 470 million berries, Kid is not a pushover, and the fact that he fought Kaido even when his back was against the wall shows how strong-willed he is.
According to Kaido, Kid also possesses the power to use Conqueror's Haki, which implies that he has the qualities of a King. Kid may not be as strong as the Four Emperors yet, but he'll undoubtedly get to that level in time, and hopefully rise to their ranks by replacing one of them in the future.
7 Can't Become: X-Drake
A former Rear Admiral of the Marines who turned into a 'pirate,' X-Drake is an incredibly strong character and one of Kaido's six strongest headliners. He possesses the powers of an Ancient Zoan Ryu Ryu no Mi, Model: Allosaurus which allows him to transform into the said creature.
Without a doubt, he is a fearsome pirate. However, Drake is also a secret undercover marine posing as Kaido's underling. As such, no matter how strong he'll get, Drake will likely never become one of the Four Emperors of the Sea.
6 Can Become: Trafalgar Law
Trafalgar Law, also known as the Surgeon of Death, is one of the Worst Generation's most fearsome pirates. He's the captain of the Heart Pirates and boasts a bounty of 500 million berries. Law has served as one of the Royal Shichibukai and fought against the likes of Doflamingo and Fujitora.
He is extremely powerful—enough to defeat the likes of Smoker without much trouble. After allying with Luffy, Law aims to take down one of the Four Emperors of the Sea, Kaido. As such, he has a decent chance to rise to the ranks of these pirates at the end of the Wano Country arc.
5 Can't Become: Capone 'Gang' Bege
Capone 'Gang' Bege is yet another pirate who belongs to the Worst Generation in One Piece. With a bounty of 300 million berries, Bege has wreaked terror wherever he's been to. After joining the Big Mom Pirates, his strength saw to him rising to the rank of 'Rook'. Bege, while strong, has tried to take down a Yonko once before with years of planning, yet failed. Barely escaping from Whole Cake Island with his life intact, it is unlikely that he'll be involved in another incident against the likes of the Yonko any time soon.
4 Can Become: Edward Weevil
Edward Weevil is one of the fiercest emerging Pirates in the One Piece world. After slaughtering a bunch of Whitebeard allies across the New World, Weevil has made quite a name for himself. He's amounted a bounty of 480 million berries prior to being given the title of one of the Royal Shichibukai, which has now been revoked.
According to Kizaru, Weevil's strength shows shades of young Whitebeard, which means that he certainly has the potential to become one of the Yonko in the future of the series, unless he's dealt with immediately.
3 Can't Become: Marco
Marco the Phoenix is the former right-hand man of Whitebeard and the first division commander of his crew. After Whitebeard's death, Marco became the crew's de facto captain and led the crew in the Payback war against the Blackbeard Pirates.
Unfortunately, he, along with the crew, was bitterly crushed at the hands of Blackbeard. During the time-skip, Marco was a serious contender for the open position of one of the Emperors. But, after the Payback war, Whitebeard's territory was taken over by Blackbeard. Marco's crew disbanded and right now, it doesn't seem like he'll form another one anytime soon.
2 Can Become: Buggy The Clown
Buggy is, undoubtedly, one of the weakest characters in One Piece, yet he has quite a high status. He's a former member of the Roger Pirates and has possibly been to Raftel. It can be assumed that Buggy knows the history of the Void Century, and, on top of that, he was one of the Royal Shichibukai until recently.
Furthermore, Buggy is a friend of Shanks, which is yet another boost to his name. Buggy's delivery has a vast network in One Piece, and, while it would be funny if he became a Yonko, it definitely won't be surprising, and there are many theories that support his rise to Yonko.
1 Can't Become: Scratchmen Apoo
Scratchmen Apoo is one of Kaido's most loyal subordinates, by the looks of it. According to the recent reveal, he set up Kid and Hawkins to be attacked by Kaido and deceived them in a shocking manner. As expected of someone with his strength, he's likely a Headliner in the Beasts Pirates, and maybe even one of the Flying Six.
Apoo has a bounty of 350 million berries on his head, implying he's deadly, yet he's willing to serve others if he feels like he cannot win. As such, there's absolutely no way that he can climb to the very top and take the position of another Yonko. He may be impressive as a pirate, but he possesses neither the skill nor the influence to become an Emperor.