1. Your own and enemy units (I mean warriors) move freely on the map, but at the same time, they cannot capture a Castle, mine, or dwelling. You kill a hero, capture all his Castles, and he gets nothing. I've had 1,000,000 cases where I was weaker than the enemy, but knowing this trick, I always won.
I won't fix the other mistakes, but keep in mind – it's uncomfortable and unpleasant to read.
2. As I already wrote (before ZYbr deleted it), there is no Resurrect spell. There isn't one in its pure form. Guardian Angel only works on strong units, Call of Death is not permanent, and raising a corpse as a skeleton (or vampire, depending on the spell) doesn't raise it where the corpse is, but near the Sorceress, which is stupid. This skeleton will hobble to the enemy throughout the battle.
3. Having more than one hero in a squad is good, but when there are 4 or 5, it's complete nonsense.
4. The choice of what to build, like a vampire or a spellcaster, a griffin or a unicorn.
6. Monsters grow every day, not once a week, which is very inconvenient.
7. It would be nice to give Barbarians spells like in HeroesV, that would be a different story.
8. Campaigns. Remember the campaigns in Her3 SOD? All the campaigns were connected by a plot, not like each subsequent one is new. And they are not interesting at all.
5. Playing only online, AI is a stupid, brainless thing like a Siberian felt boot.
9. Some monsters are made terribly, for example, the earth elemental is AWFUL compared to Her3, the knight-goblin is just... what kind of goblin in any game has been strong? They needed to make something like... a black infantryman, and that would be a good match for the Champion of the Dark – COOL?
10. A Barbarian hero can single-handedly crush armies, which is too much. They should at least make it like in HerV, where the hero casts spells and can also attack with a sword, and that's it.
11. Luck reduces damage – what is this nonsense? There was nothing like that in the first version of Heroes. And whether or not morale exists, it's also complete nonsense.
In short, I can summarize what you wrote.
If you make the fourth game like the fifth, it will be a good game.
Well done. ©

