One diplomatic strategy I am currently using is to give a per-turn gold sum to a civ that you want to see expand that is your friend already... for example the mongols were stuck on about 6 cities, and they were my friends surrounded by a bunch of non-friends, I had just had my banking revolution and was at the top of the tech tree (i don't like to spend much money on research when the bar is green...), so lots of money floating around, I gave them 100 gold per turn (and conquered some cities and gave them to mongols along with some bowmen) and they quickly expanded to 14 cities and are now a power in the region balancing out my enemies... worth thinking about, rather than try to make friends with powerful civs you can just make your friends powerful...