Imo Oda is setting up a Marijoa arc where we will see old characters coming back but also new ones everybody is waiting to see.
First of all the marines have captured Sabo but also Vivi, that means Luffy has to do something, and this also will most likely lead to Dragon who must do something at this point. So we already have Sabo and Vivi re enter the story but also Dragon who needs to enter the story after decades from his introduction as a character in Logue town. Then there is also Coby capturing(?) Boa Hancock and this introduces other 2 potential charactere for the arc. Then of course there is Kuma too, Bonney who is linked both to Kuma and the WG. Then on the enemy side we have a lot of potential as well: Greenbull who needs too to enter the story, then Coby as something playing in the middle, but there could also be other admirals, introduction of Vegapunk, maybe a small role for IM or bigger for the gorosei, Lucci could act too. And as you can see there is already a lot of stuff.
The main reasons are that Sabo and Vivi need to be saved asap, Luffy can't spend weeks or more playing pirate with the guys captured, Sabo probably will have to be executed like Ace, and that will be some sort of moral revenge for what happened with Ace. Luffy will manage to save Sabo and maybe Sabo could beat Akainu to atone for his absence when Ace was executed. Also if now Luffy and his wings are dealing with a yonko and YCs the next step for Zoro and Sanji are admirals. Don't know what Luffy could face that is stronger or at least on par with Kaido.
Against an Elbaf arc I think that there is still not enough build up for it, I mean for sure there will be in the future an Elbaf arc for sure with Loki, the old giant, maybe Dori and Broggi or other giants fron earlier in the serie etc. But nothing says it has to be right after Wano (even more when Sabo and Vivi need to be saved now). Also an an Elbaf arc means 99% there will be BM as well and the character is already redundant with 2 arcs in a row.