I think slaves should be added to all orcish faction, with Uruk-hai getting snaga and rohirrim; Angmar getting men and rare halflings and elves, dol guldur getting wood-elves, wood-men, and dalish men; and of course, gundabad (and goblins in the future) should get dwarven and men slaves!
I also think mordor should get a few orc slave skins.
Elven slaves? Naw... Orcs are basically enslaved elves that became corrupted and twisted.over many long years of torture, becoming servants of the shadow. Why would sauron want to enslave more elves if he already has hundreds of thousands of orcs at his disposal? It is true that he could dominate them but he could just breed more orcs. Besides, most of the elves in Middle Earth at the time are few and far between, and in locations out of Sauron's reach, like Lindon, which remained largely uninvolved in the war of the ring, Lothlorien, Mirkwood and Rivendell, which are all well-protected and fortified. I don't see Sauron wasting resources going over to these areas to capture a bunch of elves, which would see less results than just breeding more orcs, and these things never happened in canon, as Sauron never managed to conquer these areas successfully, especially in the timeline of the mod, before the fellowship departed. As to elven prisoners already taken and dominated by Sauron, there is no mention of this happening when Dol Guldur was first sacked or any reports of attacks or disappearances from that point until his rise again during the time of the books. The same is true for the dwarves who mainly lived in fortified strongholds at the time.
I feel like the only likely race sauron would dominate would be men as they are easily manipulated and easy to sway, and also the only race that existed in large numbers and in vulnerable locations such that taking them as slaves would be feasible and would not cause too much suspicion
I agree with your point about the elves, but I think the occasional dwarf will not be far fetched for Gundabad. Dwarves are long lived and I suspect the Orcs would milk them for all they are worth before disposing of them. Maybe a very rare chance for a dwarven slave to spawn? And that slave will be a very old and feeble dwarf?