Presently we have birds, mammals, reptiles and amphibians. What changes would be required to add an additional group? Are there any exhisting vertebrates that seem to be close to needing and additional classification? Very hard to imagine anything new appearing.
Well, they’re not new, but many fish are also vertebrates.
I left fish out, but is anything new imaginable at this point?
Something microscopic, maybe? Or some extremophile in a sulphur pit?
But as far as megafauna, really unlikely. There aren’t sufficiently large undiscovered habitats.
It’d take something like Pellucidar for it to work.
Reptilia could conceivably be way better organized. Chelonians, for instance, should really be their own group, snakes and lizards another one, crocodilians a third and birds a fourth…and that’s just the extant groups. Either that or Aves gets collapsed into Reptilia and we lose a group.
That and “fishes” is not one group. Either we’re all fishes, or you should be splitting the 3 or 4 major non-tetrapod groups up better, distinguishing the jawless, cartilaginous and bony-but-not-tetrapod fish as the extant classes (it gets funkier with extinct fish) at least on a level with the terrestrial groups.
Not only that, but crocodilians are more closely related to birds than they are to snakes or turtles. There is no scientific reason to separate birds from other reptiles.