Random Monsters Map Patcher – diverse neutral armies.

Not only HoMM, but it seems like the regular map generator does this too.

But what if the author specifically wanted an archer? Without cover?:)

And in the regular editor, there are also composite armies, you just need to find the right template. There are no combinations, of course (by the way, the combination of blue druids + light unicorns would look better). And there's nothing else, especially it's bad when it puts good and evil creatures in one unit.
The idea is very good, but it's better to implement it at the template level, and not with scripts when loading the map, it's just unnecessary complexity.