Random Monsters Map Patcher – diverse neutral armies.

Then let's immediately increase the stack sizes and add a hero to the stack to avoid giving an unfair advantage.

By the way, regarding the last point, why not? In some cases, it would be possible to generate units on the map with something like "bandit leaders." As an alternative, attach a special script to the battle with a unit so that a battle with a neutral hero begins.
This should only be done in exceptional cases (how to determine this is another question! Based on the relative strength of the neutral unit?) and only with a special option, of course.