hmm I've got no genius ideas for interface changes but I'll try to think on it! If most units need to have Open Borders / Trade Pact in order to enter another civ's territory and trade, it could be cool to have a tag enabling a Smuggler unit that could enter any territory and trade anywhere If there's something like a short term trade contract or quota, maybe would be best handling it as a Quest through Events XML. All in all some Quests making you produce and deliver certain yields to certain civs would be awesome This would be especially good to make it more challenging to get minor civs to get friendly with you and eventually get them to become your Vassals. (it would be way cooler to complete some challenging Quests to woo them, rather than have them passively become impressed saying "wow orlanth is really far ahead, I think I'll give him free stuff to make him even further ahead!") If diplo relationships were influenced by quests and missions it could become an awesome hotbed of political maneuverings, where you'd have to make an effort to win their support, with other major civs competing against you for their favour.