Let's come up with an amphibian race!

If you send this to the developers, you should definitely remove the underwater level. They won't understand it. It raises many questions. It's too radical of an innovation. Besides, a huge map with three levels will require too much from the computer. Andrei, of course, can do as he wants, but that's my opinion. A third level is only possible if you create a mod yourself. As for the invented creatures, I don't think anyone should be removed. Instead, do this: let the best ones live in the city, and let the rest only have dwellings on the map (like the Wizard in Heroes 3 and the Sorceress in Heroes 4), but you could also add a summoning altar (like the one the Elves have in Heroes 4). Then everything will fit.