What would you like to change in Heroes of Might and Magic III?

Am I understanding correctly—it only installs on the licensed version?

Versions that the Add'on is installed on: SoD 3.0-4.0, WoG, TE
=> it installs.

And do many people play by such rules?

Which ones? by the online game rules? or by VT's? People play by online game rules here, on ХВ. In ВТ they play as the game allows; there, changes are already baked into the game. By the way, there is a General chat with players there where you can arrange a game. There are more people in the evening. And I want to warn you right away—beginners aren't liked there. Because noobs throw games, quit, and vanish in an unknown direction. But maybe someone won't refuse.

Of course it would be nice to install such a mod to make life easier.

It won't get any easier.

A human—they are a creature, after all—there is always a temptation to cause some mischief or cheat somewhere. :D

Well, the ND+ mod has an encrypted saves option; that's what saves online play.

I agree. I meant that if there is no possibility of playing against a human, then one makes life as difficult as possible. In other words, practicing masochism.

Why practice masochism? When you can do more useful tasks!
For example—play at a fast tempo, but without throwing the game! This is much harder than beating bots with an army 3 levels higher.
I've written about this somewhere before. For example, take a sm-template and practice. On the first week, build up, capture the secondary; on the second, collect goodies from the respawn, take the griffin conservatories, hives, and head into the treasure zone, gather wyverns there—if you're lucky with the hives and have a good army, try to break the utopia. Learn how to use "chains." That's when an army is passed from one hero to another to clear various loot. In general, clear the treasure zone, and definitely try to break the utopia on the second or third week. And go to the opponent by breaking through ГО. Killing the computer is not necessary at all. If you do all this, it will be much more beneficial than simply fighting bots while limiting yourself with various nonsense. Because this is used in online games and will be useful. The main difficulty is maintaining tempo and not throwing. This is where the ability to conduct battles with neutrals using a not-so-large army etc. comes into play. It's very useful to practice taking crypts with different armies; almost all can be taken at 111, some at 112. Maxes, naturally. Practice taking conservatories with second-week growth. Two-three stacks should fall easily. That is, you must know how—this is very important. There's a mountain of work to do.
When you feel that you're doing well and no longer need to loop, hurry up and play with real people. Because playing against a human can radically destroy all your plans and you'll have to find new solutions; there will be situations you've never encountered before.
When practicing, remember: you aren't fighting the computer, you are learning how to play against the map. Without this, it's impossible in Heroes. The computer is not a rival at all.
Watching streams of experienced players is also useful. Some things will become clear, you'll learn something etc. There's always something to watch here in the evening http://heroes3.tv/

And one should practice only at 160%—as close as possible to combat conditions.