I think there should be some super hybrids for the aquatic expansion. First should be the Megarchelon and Helicoprion hybrid. It can be called the Megarcoprion. It has whorl shaped jaws, but they have two sets of teeth, its just that they swirl around. The body should have a shell thingy on it, with the same shape as any shark, only bigger. It is high health low damage. The second is the Xinathostega. It combines the Xinathodon with the Protostega. It has a shell, but the face is also armoured, with a plate like armour that protects it. It has a beak that fuses with the armour on the face. The fins are more bony, like a reptiles.
There can also be a superhybrid called Leptosteus. It combines Leptostega with Dunkleosteus. It will be reefs, and have an armoured face, that is less slender and more of a Dunkleosteus face. It still has a beak, and armour. The tail is a Dunkleosteus tail.
I thought about it, and although I don’t want it, I will suggest it. A Dunkleosaurus hybrid. Maybe a Dunleopleurodon. It is like Dunkleosaurus, but a Plesiosaur. It has the Liopleurodon snout, except wider. It is a surfaces, but you should make it the size of a Liopleurodon and more of that shape. It has armour, and a mosasaur tail. It has a dorsal fin.