Batallion's idea allows you to sort of immigrate towards your designated capital instead of just making your empire bigger, not sure what would be better though.
I have a new and IMO better idea:
We could just set one capital in each power's home territory, so London, Paris, Berlin, Moscow, Constantinople, Rome and I'm not sure what the capital of Austria would be.
And then you can only build units when you have a capital, and you win with 4 capitals, or just 18 SCs as usual.
Come to think of it, it would be more fun if we take it a step further: you can only build at capitals!
That would make capitals crucial for everyone, and more would be better always, while SCs are still relevant for builds. We may need to do that over mail though since you can't build in foreign SCs on this site, while that is the best part IMO.