The castle of your dreams.

National castles are no longer Heroes.
That's Age of Empires, Cossacks, or if you want a turn-based game—Civilization.
Heroes already has a land of humans—Erathia. Why do we need (pseudo-)Arabs, Aztecs, Russians?
I understand the desire for novelty. I too thought about this for more than a year, but alas—I couldn't come up with a full-fledged castle.

And why do you think that if you couldn't come up with it, others can't either? Currently, "Heroes 3" is played mostly by its fans. And they have been playing for more than a year. During this time, as Mugais correctly noted, the castles became tedious, but the game itself has not yet. However, interest has already dropped significantly. New castles are needed precisely to revive that interest. And no one will judge for introducing them—3DO is gone.