Well, I could see it as a fort upgrade. I would prefer it as a power that can be cast in a range close to the fortress.
I would see Gondor's upgrade as a the only thing you buy from the fort. The elven wood could be a power that works like the Isengard terrain, but have a bigger range to allow the stealth to work.
Mordor's effect could be a timed effect. After you buy the appropriate fort upgrade, it would start effecting. Over time, it becomes would it would be. Just like Minas Morgul and Dol Goldur. But they would not stack. Say if you have the Morgul tower in an area, that would not allow the Dol Goldur effect to work. If you wanted the Dol Goldur effect to work, you would have to make the tower an a fort a good distance away from the other fort.
Angmar's ice power could slow down all hostile enemy units. This would be a power reliant on the Witch King being built, which would unlock the the appropriate fortress upgrade.
Rohan's Grassland feature would be dependent on the the creation of farms around the map. I would actually like to change it just to a speed bonus though.
What do you think