Cuz i believe Mary Geoise is Endgame Version of Enies Lobby or Whole Cake Island
Imu is Two Characters in One imo (Just like Cavendish & Hakuba), One is just a Princess & other is The Devil ruling
That Devil is gonna be like Aokiji or Big Mom, their Role is to simply provide the Arc's Time Bomb
They are Strongest there, but They are not gonna be Defeated by Luffy or anyone else (At least during the Conflict)
The Princess that is Hosting this Devil is simply like Spandam or Perospero of the Arc
Gorosei are just like Horsemen of The Apocalypse, They are like Five Vice Admirals who led Buster Call or Commanders who led Big Mom Fleets (Such as Smoothie & Bavarois), there were also Five Fleets. So Gorosei are the ones responsible for leading Imu's Invasion/Time Bomb, They can be Defeated or Fight anyone really or maybe They won't be Defeated, whatever Oda wants, depending on their Origin Story
In my opinion, Luffy's Opponent in Mary Geoise is Champion of Celestial Dragons, their Most Popular Fighter, their Top Protector, basically their Lucci/Katakuri, and that's 100% clearly Figarland Garling
So if Zoro participates in Mary Geoise War & it's not gonna be only Luffy there, then his Main Opponent gonna be against No. 2 of Holy Knights, i know people are Hyped about Gorosei & all, but just be Patient, there are many other Characters to be Introduced
And you really think Oda would start drawing Yokai & He stops at Six Only?
There is obviously an Army of Yokai or a Race of Yokai somewhere, maybe National Treasure of Mary Geoise is Davy Jones Locker that contains an Army of Yokai (Just like Enel found Army of Automata inside The Moon), and They can only be Awakened by Blackbeard's Darkness DF
Gorosei are just like those few Automata who were on Earth & went to the moon to find there is an Army like them, didn't Oda make those Automata designed after multiple Cultures? Just like Gorosei where each one looks like a Leader/Influencer from different Country.
Oda loves drawing Animals so much, He isn't gonna only draw a Yokai Horse & Boar, the guy was Fanboying over Smile Users & you think He doesn't wanna draw as many Yokai as Possible?
The Real Opponents of SHs Crew are Holy Knights (They will also Fight Marines before too)
After that, We are gonna have the Real Final War, where Imu will still join alone, and Blackbeard will join alone, and Akainu will join alone wanting to Destroy everything ... etc
But for now, it's time for Katakuri of Endgame