They weren't always the same people! Morgoth and Sauron were very different. The only soldier that they had in common were orcs. They didn't even speak the same language!(Sauron invented his own language), and Morgoth\Sauron had completelly diffenrent cultures. They might have had the same purpose, but that doesn't mean that much. Lots of countries have 'the same purpose'. Even though Sauron worked for Morgoth, doesn't mean they were the same. Really, Morgoth and Saron are as similar as Arnor and Gondor, and to a certain extant the different types of Dwarves and Elves. If you don't believe me, Il show you the diffenreces.
Morgoth
Soldier people: Orcs, Dragons, Balrogs, bit more
-Where they came from: Morgoth invented Dragons and Orcs. Balrogs were Maiar.
Major Enemy: Elves
Leaders: Mostly fallen Maia.
Architecture: More natural, with mountainss, and caves, and so on, though they also have 'normal' forstresses.
Language:NOT the Black Speach
Sauron
Soldier people: Orcs, Trolls, bit more
-Where they came from: Orcs are all thats left of Morgoths armies, Trolls were found by Sauron and 'perverted'
Major Enemy: Humans
Leaders: Some fallen Maia, but mostly orcs. All the fallen Maia are from Morgoth
Architecture: Mostly normal fortress, a few caves maybe.
Language: the Black Speach, which was invented by Sauron for his own empire.
So, really, Sauron wasn't Morgoth, but a few relics of his ancient empire that really changed over the years. Sauron and a couple of his fallen Maia were really all the people who had to do with Morgoth.
Put this into the mod, and it would make little sense. You can't have Balrogs fighting Minas Tirith, and Trolls shouldn't be with Morgoth.