My favorite theory:

The One Piece or Pluton can destroy the Red Line. Joyboy's dream is freedom and liberation and the Red Line creates an obstacle for freedom of choice. But in Madame Shirley's vision, Luffy destroys Fish-Man Island. So I think Luffy destroyed the Red Line so everyone can be free. My reasons are:

1) "The only safe way to cross the Red Line is to ask permission from the nobles."

2) The Red Line is considered indestructible.

3) It is located right above the Fish-Man Island.

And this makes all the Blues (North, South, East, and West) flow together making the All Blue in the center. Then Sanji's dream is also fulfilled.

My homegrown theory:

In Laugtale is a big cannon that kills imu as soon as u fire it, or it’s a button.

@L725 finally one that makes sense

Ikr

My theory is that one piece is gonna end on the final chapter, but that’s just a guess