You cannot make a sphere out of hexagons.
The 20-sided die, each side surfaced with hexagons, the corners being pentagons, should give a very beautiful looking map. For a bit better, project the 20-sided figure back onto a sphere for terrain purposes. There are the vertex-anomalies (the pentagon tiles) that look sort of strange.
If you want it to look even better, do away with uniform "square" size and shape. Have the "squares" vary with the underlying terrain, and be adjacent to a non-uniform number of adjacent squares. This removes the vertex-anomalies by spreading it out uniformly over the surface of the globe.
Thinking about social policies, I wonder if the tech system from Civilization (the board game (the first one)) would be more appropriate than a tree. As I recall, there weren't any prerequisites- you could buy the most expensive tech first- but each tech gave you discounts on some other techs. So most of the time you would buy the cheap ones and then the expensive ones. The main change I expect would happen with that is that people would just save up for the best policies and get them as soon as they're unlocked, ignoring the precursors- if I can get Communism at the cost of 3 normal policies instead of 4, that's pure profit for an ICS strategy. So, if something like that were implemented it would need other changes too.
You had limits to how much you could bank. The stuff you used to buy the policies (your cards and banked troops) had limits and opportunity costs and risks.
So while in general you tried to buy the most expensive policy you could, the lower-end ones where purchased because you had no choice. They also helped ramp up your economy so you had the "bank capacity" to actually buy the more expensive ones.
I also wonder about making some base terrain tiles that are superior to the grassland/plains/hill trio. This may just be because I do a lot of playing on real Earth maps, where someone like Arabia has room for 1-2 cities and someone like Russia has room for 20 (yay Mercator projection), and appear to have a fetish for single-city nations (Although England or Japan, with 2-3 cities connected by railroad, are also pretty fun). That way you could have someplace very player dense- like northwest Europe, the Middle East, and probably Southeast Asia- that's about as resource dense as wide swaths of Russia, India, and Africa. Africa already fits that model somewhat, with vast deserts (that also occupy central Asia), but that doesn't fix it entirely.
Note that non-uniform tile size lets you do this continuously. Each city location has access to a different number of tiles.
If you aren't careful, complexity blows up -- but ignoring that...
Each tile has a worker efficiency, worker capacity, movement cost, and defence bonus value.
Improvements and technologies can boost both the efficiency and capacity of a given tile.
Toss in a distance-to-city efficiency loss, which goes down with transportation technology, and you could have lots of fun.
I took two cities from both of them and soon thereafter I got two profitable peace treaties. And all the while, I was going for a science victory!
