One of the things, I found myself thinking when I was playing. Is that there are a LOT of mountains, but not only that, they are completely impassable.
Which got me to thinking. Why? Artificial reasons of the game, and ways to create some natural borders. That I get. But why make any part of the map impassable.
Just increase the movement cost of going over a mountain be something like 6-8. Or some variable that is high and possible for those with a lot of movement. As a cost benefit ratio.
By the same token, what about finding ancient dwarven tunnels, or ancient whatever tunnels that passed through the mountain. (Could put quests inside them as part of the undermap as it were.)
I'm not saying I'm adverse to them, I just think it would be some good fun to have mountains be very difficult terrain but still be passable. And that way you could also code in some new "abilities" associated with terrain and swift movement, or other advantages of being in a terrain that the person has a perk for.