Warning: I started typing, and just couldn't stop...

I think the key to doing something like this is some sort of "infinitely upgradeable" economic system.
In all current strategy games, it's standard operating procedure to max out your planets/cities at any given tech level. You build everything you can build, until the planet is packed with everything and producing at 100% of its potential. You can research techs which raise that limit, but all you do is spend a few turns upgrading everything, and then you're maxed out again. The only way to raise your max is to grab new territory.
Instead, imagine a system where you can invest in the further development of your planet, but with no upper limit. Pouring resources into building a planet's infrastructure will keep increasing its value. The trade-off is that you aren't pouring resources into grabbing more planets.
With a system like that (if properly balanced) you would have to choose between grabbing lots of territory and having it all be relatively low quality, or focusing on a small number of planets and having them all become high quality.
The problem is: If you grab twice as many planets, won't you have twice as many resources available, and can thus upgrade all of your planets anyway, and be ahead of the guy that didn't grab that many?
The answer, perhaps, is something I'd really like to see done in a 4X space strategy game. I want your homeworld to mean something. It's the planet your race spent countless millenia growing up on, developing a huge population and a massive industry. I don't like the idea that it's so easy to take a completely empty planet and quickly turn it into something comparable to your homeworld.
A way to do this:
- The amount of population you can transport to new planets is a tiny fraction of the billions of people on your planet. In addition, unless they're fruit flies, they won't ever breed fast enough to get anything close to your homeworld's population.
- Your tax revenue comes from population, so how much land you grab won't really affect it. 99% of it is coming from tax policies on your homeworld population (plus trade, tech levels, and other things not related to how many colonies you have).
- The reason you are grabbing planets and establishing colonies is a) resource extraction (after all those millenia, your homeworld is depleted),

setting up research colonies (you can't sit home and research, you need to do research "in the field" - just as we are doing in real life), and of course c) military bases (outposts, ship factories, etc.)
All of your colonies would thus still be essential to supporting the further development of your civilization, but your homeworld would be the jewel in the crown (and you don't want to lose it!!). It's your cash cow.
Getting back to the "infinitely upgradeable" economic idea, the mechanics of the game could then either let you invest resources in further developing your existing colonies to improve their resource extraction, research, manufacturing etc. or in creating new colonies. Those resources, being generated by your homeworld as taxes, wouldn't be affected by how many colonies you have. The guy who grabs lots of planets still has roughly the same financial income from taxes as the guy who only grabbed a few, and they'll be spread more thinly over all his colonies. The guy who only grabbed a few planets can focus his money on turning them all into quite powerful colonies.
So, if properly balanced, choosing either path would get you to roughly the same economic level for your civilization, but in entirely different ways. (Maybe the "grab lots of planets" strategy would have to give you a slight edge, because it's more difficult to defend a larger number of planets).
Anyway, enough babbling...