@Deicide:
I also thought, at the time, that us taking on Enies Lobby, Impel Down and Marineford would be "too early". We literally went to three strongholds of the World Government already. We trashed the judiciary island, we escaped the unescapable prison, we destroyed the marines' greatest base.
I feel Mary Geoise will provide just a glimpse of the answers that will ultimately make sense in Laugh Tales. I also don't expect the Straw Hats to completely destroy the place, only cause ruckus by releasing the slaves. And I feel it will be important for both Jinbe and Robin to be there now.
I have two guesses at what will be the actual place for the last battle: either New Marineford, as the bulwark trying to stop the destruction of the Red Line, or God Valley, as the place the World Government tried to hide its deepest secrets in. Mary Geoise, I feel, is meant just for us to see that the World Government is more than it seems to be.
The point I'm trying to make is that I don't think there's anything for the Strawhats to accomplish in Mary Geoise at this time.
Seeing the city and getting exposed to some things that will retroactively make sense when they reach Laugh Tale isn't really enough justification on its own. Going to Mary Geoise doesn't further the crew's conflict with the Yonkou and we have no indication that it could help them find the last Road Poneglyph. Without the hypothetical "Rescue X" hook, which is wholly contingent on the outcome of events that the manga has not resolved yet and thus may or may not even happen, there's nothing to draw the Strawhats in at the moment that serves their current goals.
Conflict with the upper echelons of the World Government is inevitable, sure, but at least when half of the Strawhats waltzed into Totland, they had more than just rescuing Sanji on their to-do list.
Obviously, we're both working from incomplete information, so the Wano denouement and inevitable Reverie flashback could do a lot to set up a kind of secondary motive for heading to Mary Geoise. All I'm saying is that I don't see one based on what we currently have to work with, and that what we DO have to work with suggests to me that dealing with Imu, the Gorousei, and the Celestial Dragons is series endgame material.
The system the World Nobles built up grew directly out of the events of the Void Century, which seemingly revolve around something that Joyboy tried and failed to do. All indications are that Luffy is Joyboy's spiritual successor, and that reaching Laugh Tale is necessary for him to understand what needs to be done to set the world right, whatever that may actually mean.
The Yonkou are challenges Luffy must face on the road to becoming Pirate King. Imu and the Gorousei are challenges for him to face after he has become Pirate King.
While I always reference the Sabo/Vivi/Hancock thing, I wouldn't be surprised at all if you are right there, that Sabo saved Vivi and they both were able to escape. I don't see a "save Ace 2.0" arc as likely, but still, their fates are up in the air right now and I can't figure out what roles those two characters will have.
The thing is, if Sabo and Vivi escape, then where are they going to end up? For ease of argument, I'm lumping their respective situations together, but Sabo is a wanted man already and Vivi is likely now a target for assassination, and that's not going to change just because they got away. People are going to be chasing after them. There is obviously the possibility of them taking shelter with the Revolutionaries, but the reactions that Dragon et al. had to the news of the as yet undescribed "Sabo incident" makes me think that they (or at least Sabo) may not have that option.
I'll get into more detail on my thoughts regarding Hancock's situation in a minute, but in the same way you suggest her being returned to slavery could serve as the impetus for bringing the Strawhats to Mary Geoise, Vivi and Sabo being on the run from Marines and/or Cipher Pol assassins could just as easily pull the Strawhats somewhere else entirely - my go-to has been Elbaf, since Vivi was present for Little Garden and thus knows firsthand that the Strawhats are likely to end up there at some point. I don't think a full-on Rescue arc would necessarily be in the works, but an "Our friend is in danger, so let's meet up with them at X" setup seems reasonable to me, particularly if the location in question has been heavily built up and foreshadowed as an eventual destination, like Elbaf.
I don't think both a "Rescue Hancock from Mary Geoise" and "Meet up with Sabo/Vivi at Elbaf" are likely to happen, so at least one of those is probably off the table - and as I have a clear and admitted bias (I am, after all, an unapologetic Vivi fanboy), I think the one more likely to not pan out is "Rescue Hancock".
Speaking of…
Now, Hancock…
This is where I don't agree with you. To me, the idea that Hancock's title would be lost and her secret would be out were pretty much cemented on since Amazon Lily. The story hammered down those plot points, and we left it for a decade without giving Hancock's character a resolution. All her issues are still there, she never dealt with her traumas, and the safety of her island hanged on her title, which has been lost.
I don't buy this idea that she being defeated would "undersell" her. No one is invincible, what matters is how it happens. Unlike every other Warlord, she can't just evade capture, her people need her, and that means she stays until the bitter end.
Her victory has no narrative weight. If the story goes that direction, it just makes the marines weak. They lose 1/4 of their forces off-screen, and Hancock stays on the other side of the Red Line, doing nothing until the end of the series.
Also, Koby is not the one to "send her back to slavery", because I doubt Koby has any power in that fleet. That force has probably dozens of characters above Koby in rank. Most likely, several vice-admirals, if not an Admiral.
To me, it's inevitable that she will be defeated for whatever reasons, most likely to protect her people. What I may be wrong is her destination. Maybe she's not sent to Mary Geoise as I'm expecting, maybe she's sent to Vegapunk for slavization, or maybe she's sent to God Valley or whatever. In that case, we will find her again when we go to that place. But of all locations that she may end up in, Mary Geoise is, given what we know right now, the most likely place she's being sent to.
@Deicide:
I don't understand why people are so against Hancock coming back to the forefront. Luffy said he owned her and would repay the debt one day, and they got even closer after that. The invasion of Amazon Lily was foreshadowed for a decade, and her past a slave wasn't really explored beyond explaining her motivations. She's a character waiting for a character arc, and the pieces fell perfectly in place so far. Not to mention Oda himself teasing something serious would happen to her after Wano.
Maybe that resistence comes from acknowledging my prediction that she's joining the crew? If that's so, I may be right about her character arc and yet wrong about her fate after it is resolved. It's one prediction based on another, not a single theory.
Or is it just because they don't care about the character so they'd rather skip it? Well, one's feelings about a character shouldn't make them blind to the clues being gathered. I don't care about Bonney, but I can say for sure that she will be important in the future.
So, what makes people so adamant about Hancock staying safe in Paradise and out of the story?
I don't think Hancock is going to join the Strawhats, but that doesn't mean I think she doesn't have a role to play in the story. The issue I have is that you seem convinced that the only way she can play a role is that she has to be defeated by the Marines, put in chains, and shipped off somewhere to relive the trauma of her youth until someone comes to save her.
I don't believe that to be the case.
Hancock is strong - while it's a movie and thus of dubious canon, she's strong enough to warrant a spot on the anti-Bullet team in Stampede. My personal read is that she could probably match Doflamingo if given an excuse to go all out. Shichibukai dunking on Vice-Admirals is pretty much their default means of character introduction, so while an Admiral-tier Marine and/or Vegapunk tech could tilt things in their favor, anything less than that would be a very difficult and drawn out fight for the Marines, and that's without taking Marigold, Sandersonia, and the rest of the Kuja's contributions into consideration.
End of the day, whether she can win the fight or not, I think assuming that the only narratively satisfactory outcome of the fight being that Hancock ends up in chains is selling her short. There is no reason she cannot consider running if the fight turns against her, and if the fight is lost regardless, there's far more she can do for her people with her freedom than without it. Indeed, if it's Hancock herself that they're after, the Kuja Pirates slipping the blockade and heading for the New World could take a lot of the heat off of Amazon Lily proper, perhaps buying time for an evacuation if occupation is unavoidable.
There's plenty of room for Hancock to play a role in future events that don't require her to be defeated and re-enslaved.