Ok, I've had a chance to read this through... mind you perhaps not quite as thoroughly as I should've before commenting but here's what I think:
It was floated that perhaps we shouldn't be spreading goods automatically to all cities. hmm... good idea. Perhaps it WOULD work a bit better if we were to have the 'seed good' building that gives the Good allow instead a simple specialized merchant that can be moved to another city and set up a good export to that city.
In a perfect setup, each time you build a Good producing building, it generates say 10 of these said merchants for free and you just can't build any more than that. These units must be able to somehow recall which city they are representing and when they build a Good building in another city, it sets up a gold transfer from the city it was built in to the city producing the good.
With this mechanism, we can establish the early good trade routes, volumize the system (and even possibly nearly replace resources with this by making resources basically worthless without it being converted to a good so we can volumize resources as well), and not imbalance the system when we give a good a definitive benefit to the city that has it.
Then when guilds are introduced we enter into a new stage of goods and commerce. Until then we've been limited by the fact that we can't build more of these specialized goods merchants. Now, with guilds, we can build buildings that give us the abiltiy to build these goods merchants. They also perform more like corporate headquarters but are not wonders at all, buildable instead anywhere that you would qualify to build an original goods seed building.
We don't have to make a guild for every good, mind you, either. We could find ways to generalize the system.
Unlike Corporations, who have massive sweep and vast branding identities, Guilds are there to meet the needs of the people and to facilitate trading efforts on all levels. They are not going to be unique, exclusive, or extremely competitive as the concept is more a matter of making sure bases are covered than trying to extort a monopolization over competitors.
At Corporations we enter a third stage wherin Guilds become shattered and useless as Corporations compete for control of resources for their own benefits which the nation harvests benefits from in massive (if done right) tax revenues.
Once we've established such a guilds/goods system as discussed here, we could possibly look at overhauling the corporation system entirely. But for now, we should start with this. I agree that I am terrified to even use any guilds in fear that they may clash with future corporations. I just skip using them altogether if I can.
The only thing that sounds tricky about this to me is the gold transfer when the goods trade is established... do our units currently have any coding to recal the city that they were made in and can that be leveraged into such use?
Ok, I realize my idea needs an example to explain fully:
Near the beginning of the game you build a bamboo armorer in a city that has access to bamboo (city A). City A immediately gains the benefits of a free Good (Bamboo Armor) which grants a free Bamboo Armor promotion to troops built in this city (ok this might get more complex too down the line but lets just say that for now.)
Additionally, when the Bamboo Armorer is built, 10 (or perhaps another# scalable to mapsize) Bamboo Armor Merchants tagged with a City A designation are suddenly spawned in City A. You may now send these merchants out to other cities.
When these Bamboo Armor Merchants (City A) arrive at their destinations, they may plop down a Good (Bamboo Armor) (with City A designation) into the city. This establishes that City B that just recieved this treatment now pays City A 1 gold every round. In addition, it now gains the benefit of having the Good (Bamboo Armor) but it does not enable it to now build its own Bamboo Armorer building.
In fact, we cannot build more Bamboo Armor Merchants until later down the road in technological development when we begin building guildhalls.
The Armorer's Guildhall built in City A now allows us (because we also have a Bamboo Armorer in City A) to build Bamboo Armor Merchants with City A designation. These can be sent out to establish the same basic trade routes as we had established before with our free Bamboo Armor Merchants. The Merchants are cheap to build and can spread like wildfire at this point granting far reaching wide access. If we had a Chainmail Armorer in City A we could also thus build Chainmail Goods Merchants out of City A thanks to the same Armorer's Guildhall.
Then we enter the Corporation stages of the game later on and Corporations bundle these into basic trade routes. For now, at corporations, we just switch back to the 'free good everywhere connected by trade route' system we are currently using and then allow corporations to function as they currently do.
But in looking at this in a more far reaching light, we could use this method to revamp corporations entirely into something a lot more realistic, conceptually reasonable and can be used to volumetricalize resources better. I'll keep thinking there but it would take a while just to set up the above and playtest it a bit to see what would need tweaking before revamping the corp system.