Marine HQ are simply Endgame Version of Donquixote Pirates,
They are to WG what Doffy's Organization was for Beasts Pirates,
Instead of Doflamingo, We have Akainu
Instead of Corazon, We have Ace
Instead of Law, We have Luffy
Instead of Caesar, We have York
Instead of Vergo, We have Kizaru
Instead of Smoker, We have Kuma
Instead of Tashigi, We have Bonney
Instead of Sugar, We have Tsuru
Instead of Trebol, We have Sengoku
Instead of Pica, We have Ryokugyu
Instead of Diamante, We have Fujitora
Instead of Rebecca, We have Crocodile
Instead of Kyros, We have Mihawk
Instead of King Riku, We have Buggy
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So as you said, SHs don't have any Business with Admirals, it's Luffy's Arc Alone, just like He went to Marineford Alone,
Instead, Role of SHs in Dressrosa will be Replaced by another Crew & that's Red-Hair Pirates,
In My Opinion,
Akainu will be Taken Down by Shanks (After He tries to Kill Luffy)
Kizaru will be Taken Down by Luffy (After He tries to Kill Kuma)
Ryokugyu will be Taken Down by Beckman (After He tries to Kill Buggy)
Fujitora will be Taken Down by Mihawk (After He tries to Kill Crocodile)
Other SHs don't need to Fight Akainu's Marines at all, They Missed his Arc in Marineford War, no need to add them now,
It's instead Arc of Luffy & Red-Hair Pirates + Cross Guild playing Role of Riku Family,