The fourth column in mercenary towns.

To be honest, there aren't that many high-quality units in the ncf file. I reviewed everything. I liked a lot of it. Some of it had to be reworked. But most of it remained unused. And regarding a separate fourth column, not related to the others, I agree. I just haven't found a solution yet, so I used the third column. By the way, I had an interesting idea – to remove all creatures from the first column, defining them as neutral, with the possibility of upgrading (when joining) in the city of a related faction. Leave the first two columns as the main ones, with equivalent alternative creatures, and make the third column for other units of related factions. After all, the first column becomes useless after 20 minutes anyway. No one uses it. Everyone uses the alternatives they like. Then everything planned can be implemented.

Added 6 minutes later
By the way, new neutral creatures or faction units can be tried based on the models from Heroes 6.