This suggestion is massive so make sure you read every detail as they relate to each other. I try, throughout this "essay", to make it as balanced as possible but there will be need of some serious balancing. Therefore feel free to share your thoughts on any improvements to be made.
So, the idea behind Sauron's level up system is the whole "regaining lost power" theme. You gain more troops in an attempt to bring Sauron back to his former glory. In that mindset I was thinking of a new power for the 10 speelbook power. Here's how it goes:
Grond power is moved to Gothmog's (orc) palantir and replaces the "Release the prisoners power" which is moved to the catapults' palantir as powers that reload at the same speed as his actual power. So, Catapult can now use "Release the prisoners" as a one shot every X seconds/minutes and Gothmog gains a new Lv 7 ability which allows him to permanently summon Grond at the selected Great Siegework. I suggest Grond HP and Armor is nerfed a bit in response to his downgrade in the requirements.
This means that there is a free spot for the a LV 10 power in the Spellbook. Now there's where the main working comes in. Staying with the vibe of "bring back the lost power" I thoughts we could bring back... Gothmog!! What do you mean he's already a hero? Ohh.. I see, no not the Lieutenant of Minas Morgul, this Gothmog:
(http://i.imgur.com/cXCdDcM.png)
As a Level Ten Power:
Sauron calls upon the power of times long forgotten and bring back the Lord of Balrogs under his command for a limited time.
The lord of Balrogs would be the same model as the regular but a bit bigger. He would have the same powers cause it's pretty cool.
To my eyes this would really make the powers in the Spellbook more worthwhile as for now the power of past ages is useless if you get the ring and can only be useful a maximum of once. This I believe will strengthen the offensive side of Mordor's magic end game.
I realize that Mordor is already very strong with Sauron having twice the amount of powers then any other heroes. But I believe that currently the end power are not the best as Grond can be a bit buggy and only one on the field at the time, power of past ages can only be useful once and Tol-In-Gorauth can only be one on the field at the time
Gothmog (balrog) would be a brutal offensive machine that can dig through the enemy spam in seconds, especially Rohan IA and attack buildings. This is Lore accurate as many fans have theorized based on the different facts that Sauron's goal would be in the end to bring back Melkor to rule in his place and therefore bringing back along his lieutenant.
So? Thoughts? Comments? Suggestions? Pledges to the power of Sauron :P ?