Act 3 is meant to end in great moment of drama or tragedy. The tragedy could be Oden’s flashback that ended with his death, and if Oda decides on a great moment of drama too, this could be Shanks turning up at the end of Act 3.
There are several downsides to this theory though.
Primarily, what Oda has been planning in the background of Shanks and Blackbeard. These two characters have had sporadic appearances since the start of the series, and Shanks has had eye on Blackbeard since they were kids, culminating with who I think the certain pirate he wanted to discuss with the Gorosei in Chapter 907. It could lead to Shanks’s death, and before Luffy gives the straw hat back to him, if after Wano we have Shanks vs Blackbeard for a few chapters – which was hinted at, at the end of Marineford War.
I am curious about whether Shanks knows about Rocks D. Xebec. Rocks was Roger’s greatest enemy, does Shanks see Blackbeard being the Rocks’ equivalent, and Luffy/Roger as us readers do?
Secondly, it would be Shanks’ role in the fight, which would negate the effort of the Worst Generation to defeat Kaido and Big Mom to mark a new age of piracy in the New World.
Thirdly, it may still be early for Shanks and Luffy to meet and Oda could view it this way too, and still early to return the straw hat.