Dwarves!!
Ok, I was just being a smart alek and hadn't even read the previous posts. While I agree that it'd be cool to have a race really unique to mmo's, you have to be careful too. Werewolves come to mind. It'd be wicked cool to play a werewolf, but think about it, how do you keep what makes a werewolf cool, without making it way overpowered?
An avian race would look cool, but basically, it'd still just be an elf or human with feathers right? I doubt that RoM is gonna go through the trouble to allow flight. Besides, we already have the item shop wings. Same diff.
Vampires? See: Werewolves.
A feline race? Hmmm now that has possibilities. You could make them look like werewolf battle forms, yanno, upright, reverse-jointed legs, heavy upper bodies. Weapons could be claw extensions, or cestus (armored gloves, sometimes with spikes or blades). Instead of having hair color, you could have the choice of looking like a tiger, lion, leopard... like that. Yeah. That could be cool.