Orcs are actually mer, Orismer. They're just much more... brutal than other elves as far as culture and such.
As for an extra bestial race, I agree with XbSuper. Tamriel's lore is very integrated and to arbitrarily add another race would be almost insulting.
Then again, there is the mysterious continent of Akaviri... That's home to humans, snakelike Tsaesci, and dragons.
If there were to be another bestial race, I'd prefer that it be something birdlike... That way, you'd have the mammal (Khajiit), the amphibian/ reptile (Argonian), and the bird.
Think ostrich a slightly more humanoid figure and a shorter neck...