Purpose: To create a benefit to players for mixing with other guilds. To encourage peaceful co-existence between guilds and neutrals.

Fluff: As guild states spend time in the same alliance their cultures become more like and more uniform. This merging of cultures has an effect on all aspects of society, from customs and manners to which personal communictor is the hot one to use to which brand of personal transportation is better to use. In short the farmilar is no longer exotic, and the brands that are truly in demand are often those belonging to nation states outside of normal and farmilar guild states.

Crunch: When a player allies with another player, thus opening trade with them and the players are not in the same guild they both get a bonus to the trade income between them to account for the exoctic cultural nature of the new products and goods entering their system. This bonus could be 10% for example. If the mechanics of trade income support it, tie this to players with colonies in the same system.
How will this help: This will encourage and reward those that do not wipe out every other neutral player and guild in their system. This will also provide a big boost for players that are not part of any guild because every contact they have will result in exotic goods.

Exploitable: Yes to some degree this could be exploited if guilds broke up in to sub guilds just to get the bonus. I don’t know that this would be a bad thing however but it certainly isnt how this was intended to work. Maybe you could include something in the way this works so that if two people have ever been in the same guild then the bonus they would get is effectively cut in half because their goods are not as exotic.