[Bams]

If you'd implement a system where people control multiple planets I'd suggest the following (will become a whole different strategy game btw):

You start with 1 planet and get to colonize other planets (e.g. new shipclass which can be hit by any other shipclass). Attacking certain planets with enough (expensive and vulnerable) colony ships gives you a shot at stealing/colonizing someone elses planet. It's a very different game that way, the more planets you control the faster you can strengthen, but the more you'd have to defend and would be able to lose. I think Travian has a system like this, not sure though.