Agreed. Also:
Arthas should be a playable leader unit for the Scourge, capable of leveling up to 20 with 2 sets of abilities, one reflecting his nature as a death knight, and the other his nature as the lich king (so, he would get Coil of Death, Death Pact, Unholy Aura, Frost Nova, Frost Armor, Dark Ritual, Animate Dead, and Death and Decay).
Anuāburak could have an army made entirely out of crypt fiend variants, and play almost like the Zerg Swarm. You could have a Queen as the main building which can uproot to move around and attack, or root to lay eggs like Stukovās buildings. Sacrificial Pits should feature strongly in this, but instead of turning workers into shades, it turns any unit into elder voidwalkers, who serve as the primary casters. And of course, Nerubian Towers everywhere.
An interesting option for the Night Elves would be to introduce a rogue demon faction fighting against the rest of the Legion, and have them form a Satyr army. Satyrs would largely build like night elves, but lack the ancient of lore, ancient of winds, chimera roost, and altar of elders. Instead, the ancient of war can train Satyr Aspirants, who have some minor magical spells and can also summon a Shimmering Portal (you know the one, the flat purple portal that illidan uses in the frozen throne). The Shimmering Portal generates blight, and acts both as an Altar that trains Dread Lord, Pit Lord, and Eredar Warlock hero units, but also as a mercenary camp where Succubi, Doom Guards, Fel Guards, Infernals, Elder Voidwalkers, and Fel Beasts can be summoned, all of which regenerate on blight rather than at night. The Eredar Warlock hero gets Firebolt, Parasite (voidwalker), Dark Portal (two fel beasts and a fel guard who are improved with each level) and Finger of Death.