One Piece theory suggests that one of the strongest Marines might turn against the World Government.

SUMMARY

The recent chapters of One Piece have clearly demonstrated that the Straw Hat Pirates are incapable of defeating someone as formidable as the Five Elders.

This demonstrates that the Straw Hat Pirates, particularly their captain, Monkey D. Luffy, would require many allies to win the final conflict.
There is a theory that Luffy may find an ally in one of the most powerful marines who has yet to demonstrate his true prowess in the series.

The recent chapters of One Piece have strongly indicated that the Straw Hat Pirates are not strong enough to defeat someone as menacing as the Five Elders. They have used all of their might to fight the Gorosei and escape from the island, but still, they have not been able to defeat even one of the villainous members of the World Government.
One Piece | Toei Animation
This proves that the Straw Hat Pirates, and especially their captain, Monkey D. Luffy, would need a lot of allies to fight the final war and somehow stop the world from sinking. Although he has a grand fleet and some of the strongest characters on his side, there is still a chance that Luffy loses against the World Government.

However, there is a theory that suggests that Luffy might find an ally in one of the most extremely powerful marines who has yet to show his real power in the series. This ally is none other than Sengoku, the former Fleet Admiral and friend of Luffy’s grandfather, Monkey D. Garp.

The Marines’ Opinion About the World Government Sinking the World

According to thatonepiecetalk, Eiichiro Oda has not yet revealed how much the Marines know about the World Government’s plan to sink the world. Although they have shown the reactions of prominent Marine Admirals like Akainu and Sengoku, it does not explain what their opinion would be about the One Piece world sinking to the bottom of the sea.
The main duty of the Marines is to protect the world from the dangers of the pirates, who wreak havoc. However, if the World Government doesn’t have any world to protect, their duties would be obsolete. Therefore, there is a high chance that the Marines could declare a civil war against the World Government to protect the world from sinking.

Marine Headquarters in One Piece | Toei Animation
If the Marines turn on the World Government, it will put the officials of the World Government in a corner, as they will have to fight not only Luffy and his crew but also the full power of the Marines, which could prove to be a big problem for them.

One Piece Theory Suggests Sengoku Might Help Luffy

According to the theory, while the Marines might revolt against the World Government, there is a chance that Akainu might not be the one to lead this revolt. Akainu’s absolute justice prohibits him from allying with the pirates in any way. Therefore, he might side with the World Government to fight against the pirates.
Sengoku in One Piece | Toei Animation
Also, Akainu’s reaction to Vegapunk’s message and Sengoku’s reaction to it were completely different. Thus, there is a chance that Sengoku might be the one to lead the Marines in a battle against the tyranny of the World Government. The mysterious powers of his Devil Fruit, Hito Hito no Mi, Model: Daibutsu, would be a great help.

He is already a friend of Garp, whose whereabouts are unknown, and he has seen the resolve of Luffy during the Marineford arc. He might come to the conclusion that if anyone can stop the World Government from forcing their authority on the One Piece world, it is probably someone from the Monkey D. family (possibly Luffy).