I can't reveal why I think they are gonna get separated yet, I believed in the idea for long now, I only questioned how
But now Oda is hinting towards it, he wants to separate M3 at least cuz he wants to show multiple Islands at same time in preparation for Final Arc/War, and to do that he needed to create a Character with a Teleport Ability, and that's Kuma
After that, he needed to make them meet, so he turned Kuma into a slave so that Sabo sees him when he goes to Mary Geoise and then saves him before escaping
Now Oda only has to make Luffy meet Sabo again, which is why next destination is Sphinx Island or Island where Ace is buried
However Oda needs to explain why Sabo didn't directly return to Dragon, it's cuz Kuma is a Robot and they fear he leaks Info, which is why Oda made one of Revos Commanders, a Scientist
Once SHs and Sabo Team meet and exchange info, efforts to fix Kuma fail and he surprises them by teleporting M3 (Remotely Controlled by Vega), and btw this is why Oda didn't reveal SHs Bounties, cuz we are gonna see them through Kuma's Pacifista Eyes (A la Dragon Ball Z)
Zoro gets sent to Island of Hell, Sanji to Island of Science and Luffy to Island of Gods, or in other words Impel Down, Vegapunk Island and Mary Geoise (Sanji is Germa Masterpiece so Vega wann check him, while Luffy is wanted by WG or Gorosei precisely)
This is done for multiple reasons, it allows Oda to show multiples sides at same time and also allow connect everything to prepare for Final War
This will also help show us the Three other Yonko, because what will be their next Biggest Move?
First of all, SSG are created to counter Yonko, so Oda must show us their use in a Yonko Territory, which is why he made Shanks leave his and now he is going back
Shanks Territory will be used as Hype for SSG, once Shanks arrives, he gets super Angry and decides to attack Vegapunk Island. Cross Guild are creating a Rebellion + Doffy/Croco Team Up Hints, it's expected they are gonna raid Impel Down. BB on the other hand is a D, follows in Xebec footsteps and his Crew all have DFs that are Perfect for Infiltration
So while Luffy is in Mary Geoise, it's gonna get attacked by BB Pirates, Zoro in Impel Down will meet Cross Guild and Sanji will meet Red Hair Pirates. As for their main fights, King of Hell will face Magellan (I can prove it if interested), Warrior of Science gonna fight Lucci (Proven by Power Rangers) and Sun God gonna fight Kizaru (Finally getting his Revenge)
These three simultaneous Arcs will also be used to introduce Imu, Lurking Legend and other Villains I won't reveal now (Think of them as Final Villains Team, it will be similar to CP9 Reveal)
During all of this, we are gonna witness Destruction of Fishmen Island by Lurking Legend wearing the Bigger Straw Hat, Zoro gonna be a witness, and once all M3 escape their Islands, they all meet in Marine HQ where they will clash with Akainu, Fuji and Ryo before escaping (Just short clash)
All of this will end with the reveal that Final War must happen in Elbaf cuz that's where Villains are headed, so we get:
Monster Trio heading to Elbaf
Imu and his Team going there before them
Other Yonko set their route towards it too
Akainu decides it's time to join the war with all forces
... etc
As for other SHs who remained in Sphinx Island, Oda made one of Revos Commanders a Giant, so he can take them to Elbaf making them first to arrive and it allows Usopp to shine there with absence of M3 and become a Leader Figure. The other Revo Commander was given a DF that allows him to deliver messages in many places, clearly to help Oda move news and plans around, while other Commander is a Mink who can find Zunisha
So basically I believe we gonna have 5 Chapters about Sphinx Island and then around 40 Chapters for M3 Arc, leaving Oda with 100 Chapters for Elbaf, Lodester, Laugh Tale and Epilogue (Ending basically)
Sorry for long post xD