Elbaf has the best chance of being the next Skypiea, thematically.
A unique adventure of its own. And I think it will happen after WG war.
And I doubt many side factions will be present, not even Red-Hair Pirates.
What you described Elbaf as, Mariejois will most likely be that instead.
A place where all the factions gather together. An arc that also parallels Skypiea but also Marineford war.
And it is most likely going to happen right after Egghead when Elders accept Blackbeard's request to become King.
Totally agree with this, i also believe Elbaf is gonna be it's own Arc at the very end & it's gonna focus on SHs instead of other Factions, Oda can't focus on both SHs & Others at same time, hence why Mary Geoise Battle Royale is needed to deal with everything Political & resolve every Sub-Plot, only leaving SHs vs Final Villain (Story's Enel) in Elbaf (Where Oda also gets to focus on Vikings Culture, the main reason He wanted to draw this Manga in first place), which won't be about deciding World Power System, it's gonna be about who uses One Piece Treasure
As Oda said:
And finally, regarding the One Piece source material in 2023,
it’ll be a type of story that will have you going “No way!!
That person and that person are gonna be fighting?!”
If I were to give it a descriptive title, I’d call it a
‘Great Battle Royale’!! I hope no one dies~!!”
So imo, Wano was "Power" Themed Saga about Oda's Own Culture (Japan)
Mary Geoise is "Wealth" Themed Saga about Battle Royale between every single Faction left (Ultimate Showdown)
Elbaf is "Fame" Themed Saga which gonna only focus on SHs & Vikings Culture (Oda's Main Reason for drawing this Manga)