Luffy is going to prove Madam wrong that her future telling aren't always 100% when it involves Luffy.
anything can go down. The new fishman pirates might want to take Fishman Island for themselves and Luffy Stops them or Big Mam will get upset on non payment from Fishman Island and tries to destroy it and Luffy fights her. The possibilities are endless.
just have to wait till next week until we hear more about the future prediction Madam saw.