During the Reverie the World Goverment decided to abolish the Warlord system. For me the Warlord system was a holdover of the era before Sakazuki became Fleet Admiral. With removing this holdover I think it is also the perfect time (storywise) to remove another holdover of the previous Borsalion also known as Kizaru.
In this theory I will explain why I think Borsalino have to go, how he might go and who will end up replacing him.
Borsalino and the Warlord system
During the introduction of Weevil we learned Borsalino is someone who sees the worth behind the Warlord system. He doesn't care about any coleteral damage caused by Warlords as long as they fulfill thei purpose. And thats sth. that makes Borsalino different from Sakazuki. But not only Sakazuki didn't like the Warlord system also Issho one of the new Admirals was fighting against this system. For me this shows that shows that Borsalino is nothing but a holdover of Sengokus era thus he needs to be replaced to completly turning this marine era into Sakazukis era, an era of Absolute Justice. The big question now "What will happen to Borsalino?"!
The War against the Warlords
Between Wano act 2 and act 3 we not only learned that the Warlord system was abolish, but also that the World Goverment directly start a war against all of them at once. Among these Warlord is Mihawk the World Strongest Swordsman. I think most fans would agree that he is not only capable of fighting an Admiral, but also that he have to survive since it has become Zoros destiny to fight him. On Sabaody we learned that Borsalino has Devil Fruit based swordsman skills.Thus I see a chance of Borsalino going after Mihawk during the war against the Warlords. And this fight will result in Mihawk defeating Borsalino with Borsalino end up being forced to step back as Admiral. This might have different reason like honour, injuries or even dead (the latter unlikely).
Besides Borsalino loosing we also need someone triumphing. The one triumphing should become the new Admiral. To figure out who the future Admiral going to fight during this war one could do an elimination process.
According to this theor Mihawk is going to fight Borsalino and win. Besides that Mihawk should survive and not getting defeated because of Zoros dream.
Doflamingo is imprisoned (not part oft the war)
Law is currently in Wano and lost his postions as Warlord already during Dressrosa Saga (not part oft the war)
Bartholomeo is nothing but a brainless cyborg, who is most likely with Sabo atm (not part oft the war)
Buggy is nothing but a joke character and everyone knows he will fing a way to escape
Weevil, well he would be definetly a tough character and his defeat worthy of a promotion, but he was just introduced and hype a way too much to just end up being defeated before facing Luffy. On top of that with Weevil story being tied to Marco, Luffy and Rox I expect him to appear in Wano. (As mentioned in my theory The Remnants, the Scholar and the the Son)
Hancock, the last one left and I see no reason that speaks against her.
Thus I expect the one replacing Borsalino is the one leading the battle on Amazon Lilly! But who will it be?
The new Admiral
No not Coby. I know he is on the way to Amazon Lilly too, but one should remember he is currently only a captain and not leading this battle. Since there are a few knowing my Admiral Color-Zodiac theory spoilers ahead the one who should be leading this battle is Vice Admiral John Giant. Why John Giant? The reasons for him are quite simple
He is an advocant of the Absolute Justice
He perfectly fits the Zodiac "Rat" with first Zodiac for the first Giant Marine and now first Giant Admiral
His clothes are Orange. Orange is the only remaining secondary color after we got purple for Issho and green for Green Bull
He most likely based on the actor Chikari Hashimoto in his role as the Daimajin. Daimajin is a swordfighting Giant with a chin looking like a pair of balls same as John Giant. On top of that Oda is a huge fan of Daimajin series even have a huge Daimajin figure of it in his office as stated in his interview about General Franky.
His performance during the Marineford war speaks for himself. (Comparing him to Vice Admiral Ronse one might realize his perfomrance was actually great)
On top of that he also seems to have a good reputation. There exist no other Marine with as much medals as John Giant.
Admiral Orange Rat
For me there 3 very important things for an One Piece character to be able to become an Admiral under Sakazuki
one of the strongest characters in OPverse
following the absolute justice
be connected to the role of a famous japanese actor
And indeed there is a character who not only fulfills all three points I mentioned, but also is the perfect choice for the alias Orange Rat. The character I am talking about is Vice Admiral John Giant.
When hearing the name John Giant many think about him as the one ended up as hypetool to Whitebeard during Marineford War, but especially with new Wano content his loss isn't as bad as many think. The attack with which Whitebeard took down John Giant was an attack that need the combined CoA of all three pre TS Admirals.
Also comparing John Giant perfermance with the performance of Vice Admiral Ronse
Ronse weapon shatter John Giants sword didn't get a scratch. This put John Giant clearly above Ronse and it tool Vista above Ronse. Not to mention it actually took Vista and another one to stop an attack of Ronse.
On top of that there is no other member of the Marine who has more badges then him, which in my opinion speaks for him and hype his power. And he was the one commanding the Enis Lobby Buster Call, which is for me a sign that puts him above the likes of Momonga, Onigumo, Doberman, Strawberry and Yamakaji.
Strong supporter of the Absolute Justice
John Giant was actually introduced back in East Blue Saga holding a speech about Absolute Justice.
So one can say he is not only an ordinary supporter of the Absolute Justice he is an someone who is inspiring others to follow the Absolute Justice. Its actually not the only time we saw John Giant inspiring the troops. It wasn't Sengoku nor one of the 3 Admirals holding a speech right before Marineford War started it was John Giant.
The actor behind John Giant
Not only the 4 known Admirals but also Tokikake and Gion are based on as pecific role of a famous japanese actor. Thats why we have to assume that all Admirals have to be based on a role of a japanese actor. And in the case of John Giant it have to be Chikari Hashimoto in his role as the Daimajin. The Daimajin is not only a swordfighting Giant they also share one obvious detail... a chin that looks like a pair of balls...
Now coming to the alias. That orange fit is quite obvious John Giant always wears an orange shirt. But some migh ask why Rat? No it is not Rat the smallest and John Giant the tallest. Actually the Rat is the first animal of the Chinese Zodiac and John Giant was the first Giant Marine and will most likely become the first Giant Marine Admiral.
Besides that there is another thing that speaks for John Giant becoming and Admiral after the victory over Hancock. I think almost everyone knows that Hancock represents the Medusa from Greek Mythology. Looking up the myth one will figure out while dying Medusa gave birth to Pegasus and Chrysaor. While Pegasus is the famous one Chrysaor is actually interisting for this theory. Chrysaor is a Giant Warrior (and later King of Iberia) and his name means "Golden Sword", looking at John Giant he is not only a Giant fighting with a Sword he is also the only Marine with a golden coat! This can be seen as huge hint that John Giant will be promoted for the victory over the Pirate Empress!
Like the warlord system Borslino is a holdover of a previous era thus he have to go to make this Marine era Sakazukis era, the era of the Absolutice Justice. Thats why I speculated that it is the right time for Borsalino to retreat after the war against the warlord. A good reason might be Borsalino loosing against his opponent thus I expect him to go after Mihawk. On the other hand we also need someon to fill the empty spot, this should be some who will triumph about his opponent. Based elimination process the Warlord loosing his battle should be the Pirate Empress Boa Hancock. The myth of Medusa is a great hint that the one leading the battle on Amazon Lilly will be John Giant. Thus I expect at the end of this Saga Borsalino aka Yellow Monkey will be replaced by John Giant aka Orange Rat.
seiryu Marco Polo
my submission for the theory contest