Kongdej is right that trading with other factions is at some point a good thing that we don't want to throw away, there are many situations where a rade guild would be necessary. During total war, when all factions are at war, for very evil factions that are at war with everyone, in the early game when no one has enough to trade, in the early game when you don't have enough known races to trade, in the late game when trading is foolish due to high levels of economic competition, in every multiplayer game because everyone is too paranoid to trade. The trick is to identify the areas where a trade guild is needed and make it useful. Then I will identify where it gets in the way and make it less useful.
The more I think about it, it needs a physical presence. I will make it a Wildlands. It will consist of a massive fortress that is nigh impenetrable. If you have ever played Settlers of Cataan: Cities and Knights, it will function somewhat like the merchant piece. The first faction to make contact will have access to a vast trade network owned by the three houses. I forget the other two, but one is House Capitar. Assuming I can get it to work, I would like to give the Trade Guild some special trade treaties. Basically there will be three possible treaties. The first one has the best return on goods, the second is a mediocre return, the third is about the same as trading with another faction. Diplomacy skills will make the returns even better. There might also be a call for something like a continual trade for something you need in exchange for something you don't. Maybe metal for food per grain, refined crystal for horses, influence for mercenaries. It will be possible to start a caravan from the Citadel for all factions. An all roads lead to Rome effect will occur.
As to the citadel, capturing it will have serious consequences and benefits. It will be nearly invincible, but naturally a level 5 Town. It will have the best resources connected to it and some really powerful special resources from the cityhub. Conquering the Citadel removes the wildlands and sends you to permanent war with all factions.
The hard part of this kind of change will be figuring out wildlands and making a faction start as a wildlands. But, since there is already a darkling cityhub that essentially already does this, it should be possible. I will have to wait until wildlands are better explained and modding is fully opened to see exactly what can be done. Still, a good idea I think.
