1) I suggest making it so that you can only hire heroes of the same race as the castle in the tavern.
2) I wanted to create something like this, but it would fit better on your map: make an attack on a garrison with the hero's participation, i.e., the enemy hero attacks the garrison, an arena is summoned with an army from the garrison + your main hero; this would be more interesting (much more interesting :) bonuses from the hero's stats, magic, etc., and garrisons wouldn't be so weak, even if you reduce the number of creatures in them, because they become too numerous after some time).
Multiplayer: Господство, Извечное противостояние, Война за Магию, Торговая империя, Анатомия войны, Эксперимент
SingleScenario: Последний Рубеж, Трон, Элиралис, Вернуться домой!, Лабиринты разума
Campaign: Колонизация