I'd like to see a fusion of culture, civics, and religion in Civ V. Instead of having specific civics and religions to implement, you adjust "policies" on sliders. Technologies can add sliders or remove restrictions on sliders.
Example sliders:
<--Atheism----Paganism----Polytheism----Monotheism--> (Influence of "God/s")
<--Slavery-----------Emancipation--> (Individual Rights)
<--State Property-----Free Market--> (Economic Freedom)
These "policies" are what constitutes your unique "culture".
Then, similar to how religion affects diplomacy in Civ IV, AI leaders will like or hate you depending on how similar your "culture" is to theirs. Close borders will not spark as much tension between similar cultures, but cause major conflicts between two opposing cultures.
If two cultures are almost exactly the same, and relations are good, the cultures can create a temporary cultural alliance.