Hmm. Dark Elves fit too nicely in with the undead to have both races going at once. It would either be one or the other, or both together as one. Such a race would be significantly different from the elves to be worth it. Also, perhaps their magic could be different... instead of having air, water, fire, etc.. they could have kinds of darker magic?
Remember, LiTos, why the amphibians were scratched in the first place - they were going to be a never ending headache to balance. If you have amphibians, you have to have water bases, and how could those ever be taken with most of the units as we know them? If it could be pulled off it would be amazing. "IF". Also, how would the amphibian units function on land battles? Would they ever be able to besiege a normal inland city?
In my mind, list of following races should go like this:
After this, there would be many other possibilities, but... other races would be limited in their scope since not many other races have seen the development in fantasy that all of these other ones have.
"Yes... I am writing a book. So far I have the page numbers." - unknown author.