The main ones I think are the Dwemer, Falmer, Sload, Maormer and the Akaviri; Ka Po' Tun, Kamal, Tang Mo, Tsaesci.
My ideas;
Dwemer (for prequels, mods, or an unexpected return from...wherever they went).
Skill boosts - Armourer, Heavy Armour, Mysticism, Alteration.
Racial traits - Tricky, perhaps a slight resist fire/magicka constant? Absorb magicka greater power?
Attribute boosts - Intelligence, Endurance.
http://img156.imageshack.us/img156/9703/screenshot4t.png.
Falmer - (ditto - although harder than the Dwemer, a danger of making them simply mer-Nords).
Skill boosts - Similar to Altmer, I imagine. Light armour?
Racial traits - Constant slight Frost Shield? Frost damage for x feet on target?
Attributes - Willpower, speed?
Maormer - (I imagine them as the best race for a mage/thief hybrid).
Skill boosts - Illusion, sneak, alteration.
Racial traits - Not sure here, but a 30% chameleon Greater Power would fit. Water walking or water breathing lesser power?
Attributes - Any combination of Int/Wil and Spe/Agi
Sload - (the masters of Necromancy and teleportation).
Skill boosts - Large conjuration boost, large mysticism boost.
Racial traits - slight resist normal weapons, due to their chubby stature. Summon zombie greater power?
Attributes - Willpower? Large penalty to speed.
Not sure about the Akaviri races. Any ideas or improvements?