"Administrative Planet"
Only one homeworld makes for a culturally challenged empire...
When you're playing a map and you colonize a planet in a far-away star system, its allegiance is low, and has little hope of getting very high at all.
I think it would be neat, if you could have something called an "Administrative Planet"; that is, something you pay for like "Designate Capital Planet", except you have your normal capital planet and this administrative planet in the far off star system to keep culture levels up in that system.