So basically it starts of with Kanya talking about the past of the monster world. she talks about the cataclysm and all of that, and then the intro to the movie ends. After, it shows Kanya was talking to a group of monsters inside the volcano on tribal island. This interest furcorn and mammot so that night when everyone's asleep, they go and sneak back to tribal island to go look inside the volcano. While mammot and furcorn go check the volcano, their islands get attacked by Galvana? (Since people say they should be the villain if there is a movie). To help the other monsters, furcorn and mammot go back to kayna for help. When mammot and furcorn were sneaking around in tribal island, they had found a map, and gave it to kayna. She tells them that they can fight the celestials? If they can gather enough monsters. So basically kanya, mammot and furcorn go gather monsters from each category (naturals, ethereals, magicals and fires) to help go fight them.
The monsters that join their journey are Cybop and Pompom (the naturals), Clackula and Rooba,(The Magicals), flowah and krillby, (The fires), and Ghazt and Reebro, (The ethereals) While on this journey collecting all the monsters, they come up with a plan to stop the wublins, then the wubboxes, then the celestials. After they defeat all of them, they have a big party on gold island, and tribal island plays for the end credits. (BUT ITS THE EXTENDED AND DELUXE VERSION with the fire monsters, Magicals, and Ethereals)
I was also thinking of a prequel to the movie with a dawn of fire movie, explaining everything Kanya had said at the beginning of the first movie. Let me know if u agree!