ORIGINAL: Harmonious Hegemony
The way I see it you could send ships to colonize anywhere, its just that as they get farther from your empire's core and more into another empires sphere you stand an increasing chance (increasing with distance and time) of them changing sides. This would allow for fuzzy frontiers but would tend to keep an empire's core area free from interlopers.
1. I'm not sure I agree with the "distance from core" idea. If you start off close to another race - then your "core" area will overlap a lot, resulting in planets switching sides unpredictably.
2. The concept of what is "core" is subject to interpretation. If you discover a lost colony (ie: relatively large population) far from your homeworld - does a nearby new colony of another race stand a greater chance of joining you - or your colony of joining them?
I think greater detail needs to be put into what "core" means and what factors determine "sphere of influence". Bad implementation of this would lead to behaviour which could not be explained by the average player - "wtf? My 6 billion strong lost colony just up and joined the empire with a dinky 30m colony next to me????"