One thing I'd like to see would be robots and cyborgs. While not necessarily a "race", and maybe not even a playable one, I think we might could add them into the game.
Robots are autonomous computer-controlled beings created by humans. They are especially common in Terran-controlled areas - the government uses robots to do most jobs so its citizens have the time to pursue their own interests. Much of the society is provided and cared for by robots.
The Terrans have advanced the research of robots considerably, and this is especially evident in their military. Their smaller, simpler combat fighters, as well as non-combat freighters and cargo ships, are controlled by on-board AI. They are also working on robots capable of making more advanced battlefield decisions, allowing them to use robot infantry units. Prototype units are seeing limited combat with promising results.
I was even thinking we might have a race of robots, created by the Terrans, who gained self-awareness, rebelled, and fled their human creators. No one knows what happened to them; there have been no reported sightings. The Terran government chalked the incident off to "risks associated with robotics research" and declared the rogue robots destroyed when no trace of them was found. Most have forgotten about them since, but they're still out there... somewhere...
Cyborgs are humans that have had much of their body replaced with cybernetic tissue. We see some of this today (i.e., 21st century) with advances in robotic limbs for people missing a leg or an arm. In the future, we might have robotic eyes or hearts or such. Some, especially soldiers, who have had severe damage to their bodies have had a much larger portion of their body replaced, some to the point that the only organic, human part of their body is their brain - the rest is robotics (think Dragoons from Starcraft, but more human-looking). The League has been researching this extensively - controlling electronics and cybernetic bodies directly via the brain. They believe that consciousness defines life, so even if only the brain remains and the rest of the body is machine, the person is still alive. Thus, they seek to protect and extend people's lives through the use of these cybernetic bodies.
They are also focussed on remote-controlling ships using this brain-machine interface. They are now introducing prototype carriers with smaller ships that are controlled not by a pilot in a cockpit, but rather a pilot onboard the carrier controlling the ship remotely. Ongoing experiments are seeking to interface larger fighters with their pilots through this brain-machine interface.
~Brian