If there were new races, I'd say there should be 3 (one Mage based, on Warrior based, and one Thief based), and to fit that I'd say
Warrior- Dremora
Honestly, with the Dremora, they're not all insanely powerful. I don't see why they're not included. As for their initial stats, my guess would be something like +10 Two Handed and +5 Heavy Armor, Destruction, Conjuration, Smithing, and Enchanting. Their ability would be a command Daedra thing, and they'd have a natural +50 Stamina (like Altmer have +50 Magicka)
Mage- Snow Elf
Well, with the Dawnguard add on, I'd say it wouldn't be asking too much to see a larger colony of Snow Elves living in seclusion, and to see them represented as a playable race. Their stat bonuses would probably be +10 Destruction and +5 One Handed, Conjuration, Alchemy, Sneak, and Archery. They would have a natural 50% Frost resistance and their ability would be something relating to that "Snow Elf magic", maybe a temporary increase to how much damage Destruction spells do.
Thief- ???
Can't get behind the Dremora. The Oblivion Gates are closed, so it's not like a bunch of them will be piling into Tamriel from now on anyway, nor are they likely to become shopkeepers or anything like that.
Besides, can you imagine the opening of Skyrim if you were Dremora?
Hadvar: You from Oblivion, Daedra? Captain, what do we do? He's not on the list.
Captain: He goes to the block.
Hadvar: By your orders, Captain. I'm sorry. We'll be sure to send your... Oh, wait. Your remains will go back to Oblivion on their own. And you won't even really die will you? Honestly this just sounds like a momentary inconvenience. Never mind.
I suppose you could make a Dunmer, wear Daedric armor, and put red face paint on to make believe your Dremora. You wouldn't have the horns though.