2Ta Kotoraya
Yes, I remember Gorynych too. He could be used instead of the Hydra as a high-tier monster for the Swamp faction.

The Elementals could be gathered in one Castle, and the high-tier monster doesn't have to be a Phoenix; something more interesting could be used. For example, invent a 5th element (did everyone watch that?) and that's it! Elemental of Love)))

2Fafnir
Well, that's just nonsense... I heard somewhere in Ukraine (a guy from Kharkiv told me) that there was a mod for Heroes 3 (or 2, I don't remember anymore). There, one Castle was called "Matrosskaya Tishina" and the monsters were like "Ghostly Boatswain" and so on.
In my opinion, this is no longer Heroes, because the fairy-tale atmosphere that we all love is lost. Reality has no place in Heroes!!

In general, if the goal is to add a new Castle, the developers should simply hold a "casting" for monsters, announce some kind of competition for artists...
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My idea, based on Russian folk folklore:

1. Domovoy (casts curses, etc.)
2. Nightingale the Robber (like a rogue)
3. Ivan the Fool (cavalry; a large monster riding on a stove)
4. Baba Yaga (a bunch of various spells, flies in a mortar)
5. Vurdalak (a regular vampire, without a sword, but with fangs)
6. Koschei the Deathless (infantry, with the "Claws of Death" spell)
7. Gorynych (a flying hydra)

This is a mix of Necropolis, Dungeon, and Swamp...