*Obligatory political rant*
I had a thought. It would be difficult to implement and balance, but some sort of tech diffusion by conquering would be nice. Throughout history with the Crusaders attacking and bringing back Eastern knowledge, the Allies taking German scientists for rocketry and such, Persians take the idea of coinage after conquering the Lydians, and so on. This could be interesting as it isn't all that uncommon that a nation will take over another nation and adopt its superior methods of doing things. It also makes sense because, if America knows how to build railroads, and Mexico takes over New Orleans, then I'm assuming at least one person is left in the city that knows how to build railroads. Also, generally more advanced civs are stronger so it isn't super-abusable.
What I was thinking in terms of actual implementation is that a city being taken accounts for a certain percentage of that tech, say, 20%. Each battle one is 2%. It's just an interesting thought.