I would add more heroes to each castle, with professions related to the units of that faction. It would also be good to add more types of neutral creatures, for example, various dragons that have already been successful in other parts of the game, and bring back the basilisk, wyvern, and other "fun" creatures. It could be possible to introduce espionage through the tavern, which, at a high level, allows stealing resources, committing sabotage, etc.
It would be good to increase the number of secondary skills, for example, a city planner, which reduces the price in the city where the hero is located, and a reformer skill, which reduces the price of unit upgrades, a pacifist skill, which reduces the chance of successful sabotage, and a merchant skill, which reduces the price on