Relative to the galaxy core? How would you measure that?
Maybe have something like the following would do:
A ship jumps out of hyperspace (<-put anything you want there). The navigation systems scans for the available reference system (like a space GPS). If there is no one around it's impossible to find your position or you velocity. If there is some inhabited asteroid or planet, the navigation system locks on a given reference.
This could be fun within a solar system. How to go from one planet to another? It involves a fair amount of anticipation to reach a planet with the proper speed. Nowadays, the amount of energy required prevents us to achieve those inter-planet travels easily. Might be interesting to see how you could achieve that. What kind of "G" you need to experiment to have a reasonable time travel.
I don't know how much of this is playable though. And as you say this is only the beginning of what will certainly be a great game.
Last comment. "The Neutronium Alchemist", by Peter F. Hamilton, is a very interesting SF book to read that may fit very well in the context of this game. I am sure all of you read that one! If not, I recommend it very strongly.
-Gellule