A Dark Elves/Dwarves combo could be great for a potential expansion. Two new underground races, one good, one evil.
I know there's been a lot of undead talk here but maybe they could be incorporated into the Dark Elven fluff which would also help to differentiate the D. Elves from "regular" ones.
A concept: Dark Elves are very powerful but few in number compared to other races. They rely on necromancy to create the bulk of their armies while actual D. Elven units are not so numerous but are very "elite". Now as an interesting mechanic the D. Elves could have dark vision and wouldn't suffer penalties during night time (when on surface) but would be disadvantaged during daytime for which they could potentially compensate with "darkness" spells (at higher levels maybe even powerful rituals which eclipse the sun or something). So typically D. Elves would stay underground during daytime and would emerge when the night falls to attack, but they could still surprise an unwary opponent with daytime attacks (supported by "globe of" darkness spells) or by simply using their undead units (which wouldn't suffer light penalties, not all of them at least). The elite D. Elven units could be powerful Mages/Necromancers and highly skilled warriors (warrior mages?) with elegant swords and crossbows coated with strong paralyzing (and other) poisons and some interesting abilities etc.
And I wouldn't mind seeing some gunpowder weapons used by the dwarves and maybe even integrating a gnome subfaction into the the dwarven race for even more exotic weaponry etc.
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