In international market every civ can put some percentage of resource and sales can cause increase price per turn and reduce of demand cause price reduction.

Diplomacy:

Trade Diplomacy: It can have good impact on relations by good cross border or overseas trade route.

It can also have good and bad impact if you decrease price of resource on international market.

There is bad impact because civ competing in market in same resource will show negative response.

It can result in price war:  which cause steady decrease in price.

border closure: This can cause ban on foreign traders of particular civ to enter your territory.

Military: This can be great way of making influence on neighbouring civ.

Supporting Alliance: If you help ally in fighting war will have good impact. 

Standing army near border: It can have bad impact on relations.

If the civ military strength is weak will give you money or free resource and have bad impact on relations.

If weak civ have powerful ally will resist to give gift and, civ and ally will move military units toward border and can cause very bad impact on relations of two or more civ.

Skirmish: It will have bad impact and it will cause when 2 civs army units attack each other at peace time and cause damage to troops of one or both side.

Territory Intrusion: If army units of civ enter the territory of other civ without any agreement and can cause attacks causing killing of unit.

Alliance: This can also have good and bad impact on relations. I suggest to make a large group of alliance. It can appear when there is an aggressive civ militaristic or trade.

If one civ make alliance will increase relations and if civ have bad relations or rivalry  with other civ can also try to make alliance against us.

I suggest these features because it can cause  world war or great war like scenario with any nation and i players can see effect of decision on game world.

If you have any idea please share on this thread.