I have a suggestion that I'm not even sure is a good idea (maybe too complicated, maybe it would slow the game down), to do with trade—specifically, the connection aspect of it.
Basically, I find trade routes and resource connections to be boring in Civ 4. They're oversimplified and rarely come into play. The Harappan goal is the most interesting application of trade routes and city connections I've ever seen. On the other hand, Civ 4: Colonization went overboard, and is basically just a trade route simulator.
So, what if, instead of it being a boolean is/is not connected, distance played a factor? Roads would allow trade to move farther along them, just as they do with units. Resources would only supply nearby cities, and perhaps some buildings (Harbour, Market, etc.) would boost the range of resources connected to that city?
I feel like an example would illustrate both what I mean and how it would affect things.
Greece decides that it wants pigs, so it settles Olbia 1E of the hill iron / 2E of the hill pigs. Let's say that pigs have a range of 5 tiles.
They are connected to Olbia by road, but a road does not connect Olbia to Byzantion.
The pigs travel 2 tiles by road (1/2 movement), and 5 tiles by coast (1/3 movement) into Byzantion. They've got 2.3/5 movement left.
Byzantion has a market, which restores half of the resource's used movement, like how a promotion heals half of a unit's damage. The pigs now have 3.7 movement left.
The pigs travel along the coast to Athenai, with the islands doubling the cost of movement through them. They've got 2 movement left when they reach Athenai, and its market restores them to 3.5 movement left.
Greece also has Massalia, which is 14 coast tiles away. The pigs can't make it to Massalia, so it doesn't get pigs.
I'm not sure if this is a good idea. It would add a layer of complexity, with civilizations needing to ensure that markets and roads link their cities if they want all of them to benefit from resources.
It would make trade and city connections more interesting, and perhaps encourage civilizations to be somewhat polycentric—train elephants in Carthage, and spearmen in Sur.
However, I suspect it would also hinder performance, since it requires pathfinding to be done.
Just thought I'd throw it out there and see if it stuck. Some elements are already present in some form, such as some resources not being tradable until a certain point.