The Marines are one of the three great powers in the world of One Piece, the other two being the Shichibukai (former), and the Yonko of the Sea. Akin to the Yonko, the Marines are crucial in maintaining the power balance of the seas. Led by the Fleet Admiral, their strongest assets are Three Admirals.
Currently, Fujitora, Kizaru, and Ryokugyu serve as the Three Admirals of the Marines, but there are lots of others who possess the talent and the potential to become an Admiral in the future. Here are 5 One Piece Marines who can become an Admiral and 5 who can't.
10 Can Become: Smoker
One of the former Captains of the Marines, Smoker was promoted to the rank of Vice-Admiral during the two-year time-skip, thanks to the drastic improvement in his skill over time. Throughout his journey in the New World, Smoker had continued to get stronger, and after his defeat to Doflamingo, he's likely going to improve further.
Even though Smoker doesn't aim to become an Admiral and might actually be against the idea, like Garp, becoming an Admiral opens up the path to take over the reins of the Marines and change them from the inside, which is something that Smoker could certainly be interested in.
9 Can't Become: Hina
One of the current Rear Admirals of the Marines, Hina is one of the strongest women in their ranks. She possesses the powers of a Paramecia-type Devil Fruit known as Ori Ori no Mi which allows her to produce iron shackles.
While Hina is undoubtedly powerful, she doesn't possess enough skill to be promoted to the position of an Admiral, nor does she have the potential to make it. Furthermore, there exist characters that deserve the spot more than her, so even if a spot does open up, Hina will not be able to get it.
8 Can Become: Koby
Koby is one of the Rear Admirals of the Marines, and likely their most recent addition. He holds enough aptitude to be a member of the secret SWORD unit, which is led by X-Drake. According to Garp, Koby has a lot of potential to fulfill. Inspired by Luffy, Koby aims to accomplish his dream and catch all the bad pirates by becoming an Admiral of the Marines.
Seeing that he's risen through the ranks in an extremely short amount of time, it is only a matter of time before he becomes a Vice-Admiral, and finally an Admiral of the Marines.
7 Can't Become: Helmeppo
Holding the rank of a Lieutenant Commander, Helmeppo has come a long way since the beginning of the series. He joined the Navy along with Koby and trained under Vice-Admiral Garp. As such, his skills have drastically improved over time. As a Lieutenant Commander, Helmeppo is at least able to use Observation Haki, which makes him an adept fighter.
However, despite the improvement, Helmeppo is a couple of steps behind Koby, meaning he hasn't done enough to become an Admiral yet, and probably won't have the credentials to be one by the end of the series.
6 Can Become: Tokikake
Tokikake, also known by the code name 'Chaton', is a Vice-Admiral of the Navy as of now. During the two-year time-skip, his name was put forward for the open position of Marine Admiral, suggesting that he possesses skills of a high level. Chaton is competent enough to use at least two types of Haki, being a Vice Admiral.
While he didn't get to become an Admiral, the fact that he was a contender speaks volumes about his ability, and maybe with a bit more training, he can become more suited for the job. Aside from the Marine Admirals, Tokikake is one of the only two people who possess an alias.
5 Can't Become: X-Drake
Recently revealed to be one of the members of the Marines' special SWORD unit, X-Drake is also a member of the Worst Generation. Before the time-skip, it was revealed that Drake had left the Navy, but the reports turned out to be untrue. X-Drake has consumed an Ancient Zoan type Devil Fruit which lets him turn into an Allosaurus.
While he doesn't have any shortage of skills, Drake being a part of a secret Marine unit means that he can't become an Admiral. For X-Drake, being undercover is a part of the job, and doing that and simultaneously being an Admiral is impossible.
4 Can Become: Gion
Gion is a Vice-Admiral of the Navy, and just like Chaton, she was once one of the contenders to become an Admiral during the two-year time-skip in One Piece. Gion has an alias of 'Momousagi', which means 'Pink Rabbit', and aside from the Admirals, she's only of the only two members of the Navy to have an alias.
She wields the ability to use Kenbunshoku and Busoshoku Haki, as is expected of someone of her stature. Although she couldn't become an Admiral, the fact that she has enough power to be recommended for the position once means she can become an Admiral in the future as well.
3 Can't Become: Bastille
Bastille is one of the Navy Vice-Admirals in One Piece who made his proper debut in the Dressrosa arc. He wields a blade known as Same-Kiri Bocho and combines its usage with his Armament Haki. As expected of a Vice-Admiral, Bastille possesses decent combat prowess, but it falls short of making him an Admiral.
At Dressrosa, Sabo fought Bastille and made quick work of him, which shows that he is incompetent for an Admiral's position. Even Franky, someone who doesn't even have Haki, was able to hold back Bastille to some extent, giving us further proof that he's not suited for the job an Admiral.
2 Can Become: Momonga
One of the students of 'Black-Arm' Zephyr, Momonga is one of the Vice-Admirals of the Navy and also a veteran. Serving as a Vice-Admiral for a long time now, Momonga has become strong enough to stand up to some of the strongest of pirates. At Marineford, he exhibited his proficiency in his swordplay, along with two of the Rokushiki.
Furthermore, Momonga also resisted being turned into stone by Boa Hancock's powers by stabbing himself in his hand, proving that he's a resilient man. Although not quite on the level of an Admiral, if he improves his skills or eats a Devil Fruit, he can possibly become one.
1 Can't Become: Maynard
Also known as The Pursuer, Maynard is a Vice-Admiral of the Marines who was chosen to infiltrate the Coliseum in Dressrosa as a contestant to get a hold over the Mera Mera no Mi. Surprisingly, he proved to be way too weak for a Vice-Admiral as he was defeated by Bartolomeo, who doesn't even use Haki, by ease.
For someone with the level of skill as Maynard, it'd take a miracle to become an Admiral over time, and it would be better for both the man himself and the Navy to keep him as far away from the position of Admiral as possible. In fact, how he even managed to become a Vice-Admiral comes as a surprise to the fans, knowing his fighting capabilities.