Waterfall_13vasik
So, it seems like it can be done clearly and understandably.
The question was phrased as, "Can battles be made larger than 3x3, such as 4x4 or more?"
And it was already clear that new heroes could be added. =)
In the list of modifications (under the 3.1 version), there are at least 2 mods that include additional duel heroes. The mod authors just didn't explain how they did it. So, I had to experiment with the working mechanism and empirically determine the conditions for implementation. I thought it would be helpful to shed light on this issue for enthusiasts so they wouldn't have to struggle like I did.