About population boosting... while I have made a thread on prestige kingdom vs. empire (didn't see this one, but I also thought it seemed rather lopsided) there is one thing I've been considering, namely that empire is supposed to depend more on talented individuals than an educated people. This may also count for prestige - namely, royalty. Currently that doesn't work (They don't increase prestige in a city, as far as I know) but if that's fixed, then things may become more even.
Of course, perhaps empire should have a 'manor' or 'estate' or something building which gives a +100% bonus to any visiting champions' ability, wether it's a merchant, royal, or researcher.. the building would also provide a certain amount of population room, I assume. This could even be turned into a series of buildings.