Nargott
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I think that if there were a fourth column, it should contain random creatures from other factions.
Reasoning.
Arguments:
1. Perhaps these are mercenaries, but then each faction would need to have settings, for example, for the human faction, demonic creatures cannot be hired in this column, and so on.
2. This would make it possible to profitably buy heroes of other races as full-fledged commanders.
3. This could create a fairly powerful army without capturing the cities of other races; for example, Zealots and Elven Archers are quite powerful together.
1. The increase is the same for the entire row, so if the same number of Assassins run to the Goblins... It's unbalanced.
2. Heroes from other factions are already often used as main heroes.
3. Mixing armies should be done by those who capture cities, and not by those who just sit in one city or don't build in others.