Thats a good idea, I was feeling like "Morgul Tower" summon would be more appropriate
Then you could have the Morgul beacon be an upgrade.
I like the idea of the Witch King being moved here after the beacon has been added. Gorbag could just get a regular hero slot in the fortress. Shelob could stay there as well.
Im undecided what else could be added, maybe a map wide upgrade to the orc units that gives em a boost to armor and visually turns them into the Morgul orcs as long as the Morgul beacon is lit remains intact?
This way you dont have to boost easterlings and you get more of a boost to orcs instead.