There was a game called Homeworld that with its sequels (Cataclysm and Homeworld 2) created a wonderful story and an amazing universe. The games offered a plot that you could actualy attach to and drove you to not only play the game but gave you the feeling that you were fighting for a worthy cause (IE getting these people to their long lost home) which made the hours paid playing seem well spent. If this series of games were to come back my world would be a liitle bit more complete :)

New games might be possible if you could prise Relic away from 40k long enough for them to make another Homeworld.

The old games however are in rights limbo, to the best of my knowledge.