The fourth column in mercenary towns.

I think that if there were a fourth column, it should contain random creatures from other Factions.

Here's why.
Arguments:
1. Perhaps these are mercenaries, but then each Faction would need to have settings, for example, for the Human Faction, demonic creatures could not be hired in this column, and so on.

2. This would make it possible to profitably purchase heroes of other races as full-fledged commanders.

3. This would allow you to create a fairly powerful army without capturing the castles of other races; for example, Zealots and Elven Archers are quite powerful together.

You could also use related Factions for mercenaries, for the light Factions, or generally use spirits, animals, and monsters. And for Necropolis and Inferno, you could hire converted creatures. That is, you capture them, bring them to the city, convert them, and then you can place them in the column (with an irreversible replacement if there is already someone in the column). Of course, new models would be needed. But it's simpler here. You won't have to overcomplicate things. Mostly, you just need to recolor them. In mods that expand neutral creatures, almost everything has already been done, except for the Dwarves. You can use these developments.