One of my ideas, not fully formed yet, is to have the armies of Morgoth slightly 'themed'.
It comes from reading the Silmarillion, not described specifically, but it's there, and that is different armies led by different characters.
So for example, the army that sacks Nargathrond is led by Glaurung the Dragon. The army based at Minas Tirith (V.1) is led by the Necromancer Sauron.
The pics below are my Glaurung and Sauron BTW.
As I said, I not fully thought these through, but an example would be Saurons army would include Werewolves, and therefore more Giant Wolves, Wolf Riders. It would also have vampires in, thinking of Thuringwethil and big S himself changing into one.
Glaurungs wouldn't have these, but maybe he'd have more 'raiding' lighter armed/ armoured Orcs to range across Beleriand.
The Balrogs would be another General type. They would include the biggest and best armoured Orcs, and the Troll Guards, along with maybe other lesser Maiar demon types.
A smaller just Orc army could be led by a lesser Maiar, in the Histories of Middle-Earth the notes suggest some famous Orc Captains are actually Maiar taking Orc form.
These are my first ideas, happy to hear peoples opinions.