As a lover of all things Star Trek this is a pretty interesting idea.
Naturally all of the strawhats would have black goatees.
Luffy would be a ruthless genius that didn't give one shit about his "nakama". They all would've joined him for selfish reasons or forced to be in his crew. I'm not even sure the crew would consist of the same members and probably wouldn't join in the same order. If not a pirate, then a marine like Garp intended by a much more ruthless grandpa. Same fate for an evil Ace. The two "brothers" probably wouldn't even consider each other as such. Sabo would've stayed royalty in the Goa Kingdom and probably plot to kill his way to becoming king. If the same fate befalls him as far as amnesia and being taken in by Dragon he'd aim to overthrow and takeover peaceful and less corrupt kingdoms like Alabasta. He'd most likely run in to conflict with Crocodile long before Luffy. Although I guess in this universe Crocodile would be a good pirate or warlord trying to help the ppl of Alabasta against a corrupt king Cobra and pretty rotten princess Vivi.
Back to the strawhats in no specific order; Franky would've betrayed Tom ( this version being a bit a-hole) with Iceberg. Took over Water 7 and secretly built several Plutons. The construction of the sea train would've been a cover for their expanding underworld. Most likely upsetting the balance of power in the OP world. Keeping stuff in mind like Gol D. Roger would be evil as well as Shanks and Whitebeard. Kaido and Big Mom would've been oddly good. Most of the warlords would be nice like Croc, Moria, Doffy, Hancock being more naive and welcoming, I guess Mihawk would lean a bit more evil yet still being neutral, can't call it with Kuma (honestly depends how mirror reverse we're gonna get. Are Akainu and Kizaru gonna be pirates? Or are they just gonna be good marines. Cuz then the warlords would consist of a different group as well as the emperors and admirals. In DS9, Kira was a freedom fighter and her mirror counterpart was basically an oppressor but OG Kirk and Spock were bad guys but still a part of their version of Starfleet compared to Sisko's counterpart who before dying was fighting for freedom for humans under that Bajoran alliance's rule).
Anyway, I'll stick to most characters still keeping their professions just personality switches minus a few exceptions.
Robin would've been behind the destruction of Ohara. Being a real demon child who basically sacrificed her ppl in order to pursue the truth of the void century. She would've been secretly behind the drought in Alabasta. Using Crocodile to take down King Cobra in order to get access to the poneglyphs.
Chopper would've gotten his revenge on the reindeer that abandoned him cuz of his nose color and massacred the ppl of the Drum kingdom. Evil Dr. Hiriluk would've saw Chopper as an interesting experiment on how human (human being defined as the worst traits of mankind) a reindeer who ate the human human DF could be. Teaching him all that medical knowledge so he could go out in to the world and do his own experiments (torture) on mankind. Chopper basically becomes Kabuto from Naruto, Herr Doktor from Hellsing, that creepy mad scientist caption from Bleach, Hugo Strange, and well Queen. He'd probably sneakily become the physician in the kingdom serving under an evil king Dalton until he decides to just branch out and go do experiments throughout the grandline.
Nami's just really fucked up. Shes probably raised by good guy Arlong as her stepdad who escapes Fishman island from the evil super racist Jimbe and gets with Bellmere. Naturally she hates them and her town and oranges. Still loves money and navigation. She is just wickedly cruel the way she manipulates town's ppl for her enjoyment which leads to their deaths. Jumps at the chance to join Luffy after he shows up and kills Arlong and his crew.
I like the idea of Usopp being a despicable coward like Orochi. I just feels like he'd do much worse things as that type of coward. Like on Doflamingo's level. Do untold things to Kaya. Blackmail a good guy Kuro and set him up to be killed, etc.
Zoro is an ambition-less murderer. He is the one that pushed Kuina to her death after her father and his master told him Kuina would succeed him as head of the dojo and he can never become the world's greatest swordsman. This Zoro is a bit more lime that dark universe Zoro Oda described in one of his SBS. Zoro pirate hunts just to make enough money for booze so he can pass out and sleep days away. He's a vicious swordsman and killer but not much else. He'll follow anyone that can provide him with enough alcohol which leads him to accepting membership to Nico Robin's Baroque Works.
Brook's story is completely different, at least from what we know, in the mirror universe he tried to mutiny on Yorki's ship which successfully splintered the pirate crew during their time in the Florian Triangle. Brook naturally felt he was the superior musician and a jealous Yorki tried to kill him. Also earlier the crew tried to kill and eat Laboon. Now captain of the crew the same pretty much happens to them until Brook dies. All those decades of loneliness really made Brook insane and not in the playful way Oda depicts it. Now skeleton Brook pretty much joins Mora's crew while secretly plotting to kill him as he basically sails the Florian Triangle as an enforcer taking out any passing ships. Sort of like Mihawk's introduction but with more music.
Just like how regular Sand's story could be its own manga, same for mirror Sanji. In this version he and bis mother Sora are the bad ones. His siblings and Judge try to be good rulers and aspiring superheroes in the Germa kingdom and all of North Blue. Sora and Sanji aim to be dictators and feels it is their right to rule so they start wars and conquer most of North Blue. Even a young evil Sanji is pretty adept at it. Dora uses her scientific mind to experi ent on her children and succeeds the most with Sanji. Judge is pretty much a beta taking the backseat to his wife's and tries to be a caring father. Sora eventually kills him. Sanji's dream at the time is to basically become Soba Mask and conquer all of North Blue then take on the world government.
Now I could just keep going down that route but to fall in line with fate lets say Sanji still somehow ends up on a ship leading him to East Blue and with Zeff. Maybe Sora feels threatened by her twisted son's ambitions and sets him up to be killed on some ship but instead ends up in East Blue. The twist here with Zeff being a greedy ruthless pirate is that he really was hogging all that food as the two were stranded on that island. Sanji's uses his abilities and while all the food is gone he does cut off Zeff's leg. Yup Sanji's so hungry he has to commit a little cannibalism to survive and that's where he gains a better appreciation of food and dabbles in cooking. Still a pretty ruthless kid and totally not humbled by the experience Sanji opts to start over and conquer East Blue as a pirate and eventually take back the Germa Kingdom which is his birthright. Successful as a pirate from a young age Sanji then is gone of the top pirates in east blue and has had several bounties. Then he has his inevitable encounter with Luffy.
Also for another curveball I feel like Smoker as a pretty bad marine even with a logia (bad as in not even a captain, Tashigi is) would be drastically changed when he meets Luffy at Loguetown and this version would betray the marines and become a pirate, joining the strawhats.
Jimbe would basically be fishman Hitler. He doesn't join the crew. Too hateful. Killed too many humans. Basically like Arlong joining.
The protagonist in this world would be Teach. He'd be more like the Luffy we know. Just older and fatter.
Buggy would basically be Shanks, conqueror's haki and everything.
