I do not think this is needed. There are a couple of counter arguments to be made for your essay.
1. The timeline must be respected as much as possible. The Third Age mostly, with the only exception being, if there is a lack for anything better for a faction, like Imladris' Last Alliance summon. 1st Age creatures will not really work.
2. Further lore accuracy. During the 3rd age, the only directly known Balrog to be alive was Durin's Bane under the Misty Mountains, in Khazad-dum. Therefore, it will belong to the Misty Mountains faction. Just like Skeeverboy said, only Mandos can judge a soul to return, but he does not do so with Saruman, Sauron, Gothmog etc, because they are evil and they wish to destroy the order of the Valar and that's why they become wandering spirits in the void, having their souls rejected/not judged.
3. Balance is probably the most important from these three. Mordor already has an incredibly strong late-game. Grond is specifically designed to crack open the Fortresses, since Mordor's normal siege is not the strongest. Gorthaur's ''Power of the Past Ages'' is there to give Sauron some field presence on the field if the ring is disabled or cannot be found/acquired. If we were to give them even another mass-slayer, Mordor will be unstoppable.
There are other arguments to be made, but I think these will suffice.
Finally, fans theorising about what Sauron's ultimate goal would be as lore-inaccurate as it can get. The only lore to ever exist is the work given to us by the Tolkiens and nobody else. This is the harsh truth, I'm afraid.
I hope you realise that these kind of ideas are cool on paper, but they barely make any sense for the mod. I hope you can see that and I apologise if I come across as offensive.
That said, I advise you to keep improving your suggestions and accord yourself to the aims and wishes of ET, since I did like how you structured your post, demonstrating solid reasoning methods, and did not go ''please add unit x or y cuz OMG THEY Look/R so C00L'', because that's kind of what I expected when someone mentions add 2nd Age or 1st Age heroes, creatures or armies.
Kind regards.