Most forests irl that have not been cleared at all have have trees that are very close together. In forests around where I live, the foliage of the trees can block the sky completely except for small clearings.
In HnH, trees even in the thickest forest biome are very far apart, conspicuously so. Forests should be dense enough that they are hard to run through without paying attention, and while still possible to traverse with horses, it should be at a significant disadvantage. This could give players a more active form of gameplay traversing and fighting/chasing in forests, and give people the ability to flee from mounted units by fleeing into a thick forest where the horses have trouble; at the moment horses just excell everywhere for combat.