I personally like the idea of these
1 ) shape (strategic differences between maps, especially phasespace travel)
2 ) # of planets (duh)
3 ) # of planets / galaxy
4 ) space between planets
5 ) speed of rotation of planets around a galaxy (if this is even included at all... although it would be pretty sweet. in real life it would take way too long... but still) and if it actually is, just remember to make the internal planets go faster