Usopp will certainely have important development at Elbaf given his admiration and connection with giants characters and old dream to go there.
We may also see the Red Hair Pirates here.
The giant tree obviously inspired by Yggdrasil will have its importance.
Big Mom and the Charlotte Family will be present in the arc, with Linlin's ties and issues with the giants and Mother Carmel being exploited here. We may possibly see the giant family reunion and Linlin reuniting with Gerth and Hajrudin.
Hajrudin and his crewmates, in particular Gerth, will be important allies in the arc.
The last of the Road Poneglyphs or its location on another island will be discovered, another secret about the Void Century will be revealed.