I am not sure if this idea has been suggested before, but what about the idea of adding specialized attacks, resistances and weaknesses to units. Here are some examples; they don’t necessarily have to be these units, numbers or abilities.
Attack
Spiders currently have +20 attack in city terrain
Spiders could have +5 poison attack and +15 attack in city terrain
Giving them 25 strength non city, 40 strength city
Resistance
Wolf Rider +5 resistance to poison
Spider attack not in city is 20
Weakness
Crusader -3 weakness against poison
Spider strength 22 non city, Crusader strength 20 (+3 moral bonus -3 poison)
Attack
Elemental currently has +25 battle while attacking +15 while attacking.
Change to +10 battle while attacking and an additional +5 fire attack.
Elemental could have same stats, but with choice of purchasing 1 and only 1 of 4 different elemental attacks. Fire, Wind, Water, Earth
Resistance
Wizard +3 resistance to fire, wind water, earth
Resistance & Weakness
Yeti -3 weakness to fire, +2 or 3 attack ice replaces or coexists with +2 fear
Something like this could add another dimension to the strategy of offense and defense and add to the list of abilities a hero can spend points on.