Symmetry in diplomacy is on my list of complaints and will probably come up in a future CIV ideas thread, but here's an
early version I posted on the last year. There's a lot more there than just DOF as well.
As for my ideas not worthy of a thread, I would pick 'map trading'.
There should be a nice middle ground between trading as it existed in Civ3/4 where you can basically sit in your capital and see the world, so to speak, and the claustrophobic no map trades in Civ5 where in many games, you have to wait until satellites to see the whole map.
Scouting in the early game is just tedious and it's impossible to recover /stop build orders to replace a lost scout halfway across the planet. Some diety level strategies actually suggest restarting the game if the scout is killed. That is kind of extreme, but it underlines how the game has shifted heavily in one direction
So my suggestion is to allow 2 types of maps to be trade-able
Classical/Medieval era allows the trade of topographical maps showing general topography of their explored areas. So you will see features like mountains, plains, hills, natural wonders (this will count as you finding them), but not cities or who is near them, though I suppose you can guess it, or resources/luxuries location. This will also be a nice incentive to explore.
Maps will be tagged with number of natural wonders in it you haven't discovered yet and you can trade those for more. Similarly the AI will ask for more to trade you their maps with wonders.
Renaissance Era allows the AI to trade you a more detailed version that shows their cities and resources as well as city states, but you will need to send an 'embassy' to meet those CS. revealing them on the map will not count (I believe this is similar to how satellites work) ; if you haven't traded any maps to this point, you will need to trade the topographical maps first, then the detailed version.
Furthermore, while any AI can trade their combined 'world view' of their topographical maps, they can only trade you the detailed maps from their own cultural line of sight. So no more exploiting that 1 uber trade where you trade 1 map to show you the entire world in its full detial.
Again, amount of resources they see vs. the amount of resources you don't know about and will potentially see will affect pricing and this correctly prices the cost of information for a large empire's territory. So I can expect map for luxury or map for 1,000 gold being possible in some trades.
And both civs will need 'Astronomy' tech to trade the more detailed maps.
After this point, satellites should be a modern era 'top off' to reveal bits of fog on the ocean or the territory of your hated enemy who refused to trade you their map or that large super empire who you didn't want to shell out 2k gold for.