giving Dark Magic to Druids – I think it's just a sacrilegious idea. They have insane spellpower, they have immunity penetration, they have elemental chains, they have Arcane Wave. They have achieved perfection of body, soul, and chaos. To give them some kind of dark crap is just to waste a skill slot, which you'll never use anyway, unless you enjoy masochism, of course. What's better – to, uh... hit with Earth Shock/Rain/other nonsense or cast a debuff on the enemy that won't play any role when you... hit him with Earth Shock?
Necropolis can quite easily have both Darkness and Summoning, and they often do, and the result is clear – they raise units excellently and crush everyone. They're satisfied and content – what more could they want?
as for Dwarves, I would definitely give them Light, especially Helmar.