I suggest the culture of "La Serenissima", The Republic of Venice
Why this culture?
The Republic of Venice was, by its time, one of the greates traders in the world. In the recent 12-18 months, the game has leaned strongly towards favouring fighters - especially in GBG, negotiating makes little sense once you are in Platinum or Diamond league (Sectors are taken within 1 min or less). Hence, I would love to have another settlement which gives bonuses to traders, as I think the game should give a REAL option to choose between trading and fighting! It makes sense to let a trading culture give buildings that favour trade and negotiation.
Setting
Obviously, we are in the Venetian Lagoon, and we have to bring the Floating City, after a strong Agua Alta, back to its flourish. This is done by the all-known procedure of building houses, producing cultural goods and going through the questline. There would be two possibilities of how to obtain the Cultural Goods: Either by the "normal" way in which houses and Diplomacy Buildings make Florins, which we use to make the goods as usual, or we need to obtain some "Trade resource" by three negotiations per day, similar to the fighting of the Egypts. I would grant access to this Cultural Settlement in Late Middle Ages.
Goods could be Venice Masks, Murano glas chalices, Artworks (Venice was a centre of arts and writing by its time) and some simple good such as fish
As for the mini game, I imagine some sort of labyrinth (have you ever been to Venice?! It is INDEED a huge labyrinth once you leave the main tourist paths). There was an Agua Alta and everything is flooded, blocking the direct path to where you want to go. Instead, you have to take sideways and sometimes cross pieces of water. For that, you get a fixed number of bridge pieces. To make it a bit harder, there is a timer and you have to find the right way, inluding the right places to put your bridge pieces, in that amount of time. DIamonds would be spent to get another piece of the bridge.
The entire settlement takes 10 turns to complete the main reward, leaving 3 turns for embassadors (please, do not make them give medals or supplies...)
Reward
Main reward: Doges Palace. 4x4, 7 levels. At max level, it gives:
Coins and Supplies depending on Age
25% each of Supply and Coin boost
75 Goods
12-15 FP
20% chance to retain the goods spent on a negotiation -> This is the big turning point in making negotiations a viable option for GBG again!
Time reward: Murano glas maker, 3x3, 5 levels. At max level, it gives:
Coins and supplies depending on age
10 goods
4 FP
2% chance to retain the goods spent on a negotiation
Side remarks
GBG have pretty much changed the game. Big players receive 1/3 to 1/2 of their FP income by it, and this is all thanks to the possibility of making near limitless fihts, provided there are 4-5 siege camps. However, even if I have 5 Siege Camps, I cannot negotiate forever, as it will still require goods. I have understood that Inno does not introduce an auto-negotiate feature (God alone knows why), so this wpuld be an alternative to it. Otherwise, there will hardly be an advanced player who really builds its town towards trading!
An alternative to giving a chance to get back the goods spent would be a chance to not use up one trial, essentially giving a chance of an extra move in all negotiations.