Yeah, the biggest problem is that we don't even know if there are any other islands, and then there's the fact that no Matoran are from other islands.
And now six warlords fight for it to seek an escape from their banishment.
Wow. Stellar performance. Slow clap.
Oh yeah, that's another thing. Seeing how the Elemental Lords are not on this island, it wouldn't have elements to begin with.
Now that sounds very time-consuming...
Okay, so the major issues with this are the ideas that these Matoran come from a different island that you've already talked about being banished and such, so I'll leave that alone.
Second, another island may not even exist, though I'm sure TTV could change that if needed. If not, perhaps the Matoran could have built a floating prison like Alcatraz, and common visits by Tren Krom could have forced all of the guards out.
Third, if this other island did exist, it would have no elements, so leaving it would change nothing. Perhaps they were banished from Artahka, but as they spent time away, their masks broke off and elements drained, turning them into big-mouthed monsters with no sense of morality whatsoever.
Fourth is definitely Tren Krom. I think if the Toa were fighting him underwater, and his priority was to devour the island, he could just swim over them, break off any feeble attempts at slowing him down, and keep going. I would suggest giving his motivation to be mainly his primal instincts, guarding territory, preying on food, and defending himself from attackers. If he is looking for food, and is attacked by the Toa, he would break off from the hunt and protect himself as well as squash the Toa.
Fifth, the Piraka's individual names are canon, but not the word "Piraka". I love the idea of Piraka being pirates, and I like the pitch so far, so with some tweaking, this pitch could be amazing!