I'm just hoping with the mountain-walk ability it'll be possible to code in the changes I want for my game... currently no matter how you try to change the Mountain terrain feature, it appears to be hardcoded as 1 movement and impassable (I tried making it so barbarians could traverse mountains but it ended movement but can't get it to work - works fine for ice, gave Norway the ability for all naval/embarked units to traverse ice but the movement cost was 10, and wonders, adjusted Old Faithful to be a hill and Potosi/Fuji/Krakatoa to have a movement of 10 and be impassable without promotions, but no luck with mountains apart from a Can Move Impassable letting you cross them with a move cost of 1 - but that allows crossing ice, wonders, and all other flagged-impassable features).
If these abilities work I won't have unbeatable cities in my games since some late game units (and early barbarians ones) will traverse mountains (added a Combat Engineer that is a recon class to the Globalization tech - all it previously had was the UN. Now it possesses a low CS - late modern unit with late renaissance unit strength - worker than can move impassable, paradrop, and gets scout promotions)