Since others are suggesting factions, I thought I might chuck some in. Not saying the game needs them, just having a bit of fun.
The Gwathlo Men, a faction stretching from Lond Daer to Royal Tharbad along the western bank of the Gwathlo river in southern Eriador. They could be remnants of both great-but-ruined Numenorian cities, mixed with travelers, wild men and Cardolani who fled to this area for safe haven. Like the original Umbar faction DAC had, they could be a mix of pirates, bandits, farmers and fishermen, mercenaries and merchants, and other odd folk simply trying to get be, like a rougher version of Bree.
They'd be a little less wild than their neighbours, but a little more martial than Bree. They might spice up the Enedwaith-Dunland gameplay, which for a human player always starts with swallowing the other, by necessity. It would create a danger factor for Bree and the Dunedain, presenting a higher chance of having to fight on two fronts. They might help keep Lindon off of Enedwaith, which has always seemed just cruel and illogical to me - Elven genocide of forest people simply because they're bored.
Like Enedwaith, the men of the Gwathlo would have no allegiance, meaning every game would be different.
The hard part would be making the soldiers unique while still incorporating elements of the composite cultures.
Another faction might be Goblin Town. As seen in books and films, these guys have their own king and culture, separate from Moria, and hold an incredibly strategic point (the High Pass). You would have a faction capable of striking the good factions east and west, which is something Moria struggles with, especially since they have Dwarves bordering their capital. You could make them a mostly jav-bow ranged and stealth faction, and essentially unarmoured/naked, with monsters like trolls and wargs as elites, and maybe some unique monsters - there are all sorts lurking in the mountain's deeps. You could also have more powerful orcs come in from abroad, essentially being strong-armed by other factions - like Moria is in the books. So a Goblin army might have 1-2 rare Isengard, Mordor or Gundabad swordsmen, a higher number of monsters (like wargs and trolls) than other orc armies, and then the majority hit and run cowardly goblins. Since they are wilder, you could also have them unallied, or loosely allied, meaning they could end up fighting the other orcs of the mountains.
They'd be different from Moria in that Moria has a completely different aesthetic, more dependable (but still unreliable) soldiers in their iconic Balrog armour, and a more savage and militaristic feel to them (Moria has got to be decent to fight Dwarves toe-to-toe).
I'd also suggest pushing the Dunland province of Byrig up against the mountains (they are the ​hill​ people of Dunland), and maybe making more goblin settlements in the mountains which aren't passes, so it is easier to walk between and defend the mountain pass provinces, and it feels larger and more rugged, with more mountain battles. My guess would be that it might make path-finding for the AI easier. You could also make the Gladden Pass have no settlement, which lore-wise is correct (it's on Caradhras, a huge mountain peak), and would make it easier for cross-mountain travel, and more fun to defend, since it isn't just another siege.
​Completely​ unrelated, could Harad have some AOR/merc access to corsairs? Harad straddles the river Harnen and border Umbar, it would be cool to have some access to different looking but lore-friendly units.
