I think it will have to do with the eve tree.... As things are currently in the manga events are escalating quite fast into something which will affect the entire world. Luffy already declared war on big mom and he is actively trying to take out kaido and bring the entire world into disarray by taking out doflamingo. I think that eventually luffy will have to go to majiroa and there he will somehow endup destroying the eve tree. Once the eve tree is destroyed fishman island will become uninhabitable. perhaps even the bubble around FI will become unsustainable. Once that happens we are talking about a mass exodus from the bottom of the sea at which point the sea kings will carry FI population away on noah to a new kingdom.