Mod requests (orders).

by the way, maybe it's time to stop thinking like that.

firstly, you can create as many new cities as you want that won't replace anything.
I mean, these cities will have a unique appearance, building tree, and recruitable creatures. The only thing you can't do is give them a new, unique racial ability (but you can simply not allow them to build a structure responsible for the racial ability).
and most importantly, you can't add such a city to the map selection menu so that it can be used on par with the other eight races. But such a city can be easily placed on custom maps in the editor. I actually made such a Naga city in the mod that DarkenRahl785 wrote about.

secondly, there is another feasible idea that can now be normally integrated into the regular Gameplay; this is alternative buildings.
I described the theoretical idea of its implementation here long ago: /topic/at/1009839/
Now, after testing, I can say for sure that this scheme of alternative buildings works. By adding a couple of visual features to it, you can get almost a completely new game race.

so, perhaps it’s not worth saying “almost impossible” so categorically. In principle, with clever manipulations, it is possible to do much more things with heroes than we are used to thinking.
I recently described the idea of an external program - a visual creature constructor. And just recently, Esto100nec seems to have figured out how to import animations into the game. I haven't had time to test this import myself, but if everything works, then this feature of adding new animations to the game gives much more graphical possibilities than it seems at first glance.

in short, you can do very, very many things, but implementing all these ideas in a normal way requires a lot of time, and there simply isn't enough of it.