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I'll try to suggest some of my ideas; maybe you'll like something:
Part 1. Units:
1) Increase the damage of Imps and Goblins to 1-3.
For all other Tier 1 creatures, either set the upper damage limit immediately or have it become 3 upon upgrade, with the exception of Blue Wizards (but they are ranged units).
For Goblins, if possible, make it so that their damage is 1-3, and upon upgrade, it becomes 1-2 (but they gain ranged attack).
2) Increase the damage of Orcs (Orcish Leaders) from 2-5 to 3-5.
3) Add the same ability as Minotaurs to Sailors and Upgraded Sailors (always positive morale).
These minor changes will affect the possible starting options for these factions.
Part 2. Objects
1) My main idea is to add "fortress-like objects".
That is, a catapult (siege weapon) can be used not only when capturing cities but also for some new objects.
These objects will consist of a mixed army – that is, ranged units + melee units to protect them from being overrun.
In battle, there will be a wall + 1 tower, just like in sieges.
These objects will have 3 levels of gradation.
1.1 - Outpost (Nymphs + Lizards), possibly with upgraded versions.
They provide 2 types of specific resources (Sulfur + Mercury +5/7/9), Gold 5k/7k/9k, and 2 artifacts: Treasure/Minor, Minor/Minor, Minor/Major.
1.2 - Sanctuary (Liches + Necro-Steeds), also possibly with upgraded versions.
They provide 2 types of resources (Crystals + Dragon Scales +6/8/10), Gold 8k/11k/14k, and 2 artifacts: Minor/Major, Major/Major, Major/Relic.
2) Adding and changing objects where you can obtain random scrolls of certain levels.
2.1 Cathedral - (Monks/Fanatics/Wizards) - arrangement as in Confluxes (Fanatics are placed in the upgraded position - lower left corner, Wizards are placed on the right in the middle).
Also 3 levels.
They provide scrolls of levels 3/3/4/5, 3/4/4/5, 4/4/5/5 - and you need to add an option that a control scroll can also drop randomly here.
2.2 Change the Pyramid so that it provides a level 5 spell scroll instead of just training the hero who defeats it.
3) Add an object where you can advance a hero's skill for a random resource.
Alchemist's Laboratory - for 10 resources of a random type, the hero can advance an already learned skill once per game (from basic to advanced or from advanced to expert).
Part 3. Other additions
1) Since Necromancy has become "devalued," give heroes of the Heretic (Inferno Mage) and Alchemist (Tower Warrior) classes a chance to learn the "Necromancy" skill. (If I'm not mistaken, this was possible in the first version of Heroes).
2) Add a major artifact.
Boots - reduce the movement speed of enemy units by 1.
*I also have ideas for changing underused skills, but I think you get those suggestions often.