Or... How about you could see the directions of the enemy?! You know, like when you press Z and go into Command Mode you can see all the paths your units will take. The Mirror shows things in the future, so she could "see" what the enemy is about to do before they're even there. How about that, no?
While I think this would be awesome, I'm not sure that it would be entirely possible. I also think that revealing the whole map might be a little op, because, as you said, the Mirror only shows one possible future.
The ents have pretty much been removed from the elves, replaced by the Noledhil. So long range fortress defense expansions would be some form of Noledhil platform/ flet.