What would you change in Heroes of Might and Magic IV?

It just doesn't make sense that there's no first-strike advantage. In real life, when one infantry unit attacks another that's on the move, the first one has the advantage, but this isn't reflected in the game at all. However, if the second unit is prepared for an attack (defense), then it's unclear who will strike first. The first scenario is accounted for in H3, and both are accounted for in H5 (the "Readiness" skill). This is a useful tactical feature that's built in by default, not "optional."
But in H4, combat turns into a bloodbath, and to avoid this, you have to take warriors (e.g., Nomads) or spells (e.g., Serpent's Strike), although in reality, this only reduces losses, but rarely affects the outcome of the battle, as the damage isn't high enough, and you can't eliminate an enemy unit of equal strength in one turn. As a result, these "tweaks" only help reduce losses from neutral creatures, which raises the question: are they even necessary? In H5, it might still be somewhat useful: you fight neutral creatures, reduce losses, go to a mentor, and remove unnecessary skills, retraining warriors into alternative, more powerful upgrades, but without the first-strike advantage. But there's no such system here...
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This is by no means an indicator. The 5th and 6th games are aimed at the masses, and for the most part, they are infantile and simplistic. Is having 4 resources really an expansion of tactics? O_o

I wouldn't equate these two games. The 6th game is known for changing everything that was old and good (by the way, the 4th game is also very similar, not in terms of game mechanics, of course, but in terms of its attitude towards the game and what needs to be changed, and what doesn't).
The creators of H5 managed to introduce innovations into the game without destroying the best aspects of the Heroes series. I won't even comment on the focus on the masses; just compare the number of fans of H5 and H3.