I believe Momo is the literal key to opening Wano's borders. Whoever receives the One Piece must need Wano's assistance, but Wano can't help if the borders are closed.
I've liked the suggestion that Wano is actually a turtle placed in slumber, and must be awakened by a certain someone. Zunisha couldn't even attack Jack unless Momo gave the order, for example.
Zunisha was also ordered to roam the earth and never stop. A turtle placed in inactivity for a long time by the Kozuki of old sounds, plausible when you consider Zunisha's predicament.
Plus Genbu, the turtle this hypothetical Wano turtle would be based on, was encroached by a serpent. Likewise, Wano was figuratively encroached by Orochi, whom suffocated the country due to his rule.