I don't remember the Palladium game including any fish or pretty much any aquatic critters (except for the turtles themselves and a few amphibious critters or water loving mammals like otters...not even dolphins).
The rules also present the concept that all mutants start as animals and follow a path toward humanity with their mutations...This leaves out certain elements that were distinctly a part of the Turtles TV show.
April O'Neil transformed into a fish mutant in the episode "Rebel without a Fin"
This is something that could never have happened in the original RPG.
Similarly, from the comic book...
...but the game doesn't allow for invertebrates (except for insects in the much later sourcebook "Mutants in Orbit").
There have even been a few storylines regarding unstable mutations, while the original game works on the assumption that once animals have mutated into their character forms, they are basically locked permanently into this new form. I;d be more interested in saying that the characters have stabilized into a new form, but further radiation/alien-attacks/magic/weirdness might prompt mutants back to their original form, might force them toward a more humanoid form or might do something else entirely (short term or long term).
It adds a whole lot more scope.
Further research has even shown that there have been plant mutants in the setting...but that's something else entirely.
So much unexplored potential that can now be considered if the majority of human equipment and clunky combat system are eliminated.