Quick theory: With the Candy Factory being partially eaten in this chapter people are speculating Big Mam might appear. I think she'll save Fishman Island from Luffy's destruction. In the past to set up the status quo for strength, Oda's had characters preform feats and gotten there ass kicked.
To show how powerful the warlords were, Zoro fought Mihawk who used a knife to fight.
To show how powerful an enemy like Crocodile was, Luffy was beaten, twice!
To show how some Devil Fruits can counter others, Eneru was thrashing everybody until Luffy arrived.
And then when we were really becoming conscious of the strength of the most powerful ones, Aokiji took care of the Monster Trio with ease. Which led to Usopp realizing how weak they were compared to the real strong people in the world.
After Enies Lobby, every arc seemed to show how little they grew in retrospect to how powerful the REAL pirates and Marines were. Luffy was completely drained after fighting Moriah. The entire crew and Rolling Pirates and co. couldn't take down Kuma. A Pacifista nearly took down the crew, and Kizaru almost slaughtered them. Then we get Impel Down and Marineford where Nyon's phrase "He's like an ant in a storm" really became clear.
Now we are at a weird gray point. We know Luffy could take down the Pacifista's from years ago with ease. We've seen the crew doing amazing feats, but we don't know how strong they are in retrospect. We purposely got a villain who may have posed a big threat before the timeskip with his own strength(Vander) and an enemy who has the option of multiplying his strength multiple times, and we haven't seen him succumb to any side effects of the pills yet.
I think Big Mam is near by, and by the end of this arc, she'll arrive. I think Luffy's gonna destroy the island by destroying the Tree of Eve, which gives light to the Fishman. If Luffy destroys it, not only will there be no light, however the tree probably goes threw the bubble. I'm assuming it reaches the sunlight somehow(Another theory is that it goes threw the Redline and up to the top near Marejois to get light). If Luffy destroys it it may leave that hole in the bubble where it was to suck in water. This would cause the rest of the bubble to pop, and the island destroyed.
But wait! Who was shown to have a plant on their flag: Big Mam. Who do we predict might have some Devil Fruit that has to deal with plants: Big Mam. She might fix the tree as Luffy destroys it at the end of the arc. We don't know her motives at this point, but we know she wants her Candy Factory(Pills or just candy) and she MIGHT have a plant fruit, and the tree is shown to be very important.