I have a problem with global organizations in fantasy eras. Now, this is going to sound hypocritical because I have global organizations in Fletnern, but I think that it takes something really powerful and special to have a global organization.
First - Why are they a problem? With the ability (or more importantly the lack of ability) to communicate over long distances, how do these organizations organize? How can the “mages guild” have chapters in every city? OK - They can have chapters, but they should not be seen as a global organization, more like franchises of major organization based somewhere else.
It really is this communication issue and the lack of organization that I think is the problem. If you have a “global organization” but they do not communicate frequently, enforce their rules, and constantly maintain a strict, common code, then it isn’t really a global organization and shouldn’t be considered all that global.
Some examples of how it can work: The High Order of Telepathy in Fletnern - These guys are all mentalists who can communicate across major distances. They act as a style of telegraph office for sending messages. But before you say I just wiped out my argument about any global organization communicating, the messages are like telegraphs - 10 words per message unless you’re willing to really pay through the nose. Also, each relay telepath gets the message and then passes it along - so no secrets can move this way. Further, there is a central school where most of them are trained, and they have a secret police force that can easily look into your mind and determine truth and intent, so you really can’t just trick them. They have the ability to control their own communications and enforce their rules. These guys work.
The Roman Catholic Church of the past - Global organization, right? Well, yeah, but how well did that work for them? Even with the Pope sending envoys and other guys out to try and keep the radicals in line, all sorts of nonsense happened. Now your modern Protestant will likely blame these on the church as a whole and forget that they too were part of that church at the time, but that’s beside the point. I bring this up because I think it is a fairly good example of how even the most powerful organization in the Western World at that time was unable to truly act as a global organization - way too much variation from country to country and lots of in-fighting. No, I am not saying they are/were evil, just unable to maintain a global organization with the level of technology they had at the time. For a global religion to work, the divine would need to be extremely actively involved, basically handling communications himself (sending angels down and stuff like that, not just to a prophet here or there, but to pretty much every bishop or regional leader). Hey - It’s high fantasy. You can do that if you want.
The Cartographers Guild - This is less a global organization and more like a loose federation. It works because map makers have a tendency to travel, so they can handle their own communications. They share their maps and try to go for honest and truthful maps. When they find a cheater (someone selling false or inaccurate maps), they report that guy to the local king. They cannot enforce their own rules, but rely on others to do it for them. Not only that, but they tend to inform the local king by letter as they are leaving town, just in case the king happens to be the map maker’s brother in law.
Slavers’ Guilds - This is the thing I really hate. These occur in literature, and they give these guys nearly omnipotent powers. They don’t do it intentionally, but they do give them extreme powers. Yes, slavery should be a part of fantasy adventure. It occurred too often in history to be assumed to be rare. Yes, they travel around, delivering slaves to markets all over the world, so you can say that they might be able to communicate. You might even think that they could have enforcers who audit slave auctions and make sure no one is cheating (however that works with their rules). What I really object to is the idea that the slavers would/could all work for the same global organization and in some fashion pay taxes and all work together. Slaver traders might need some level of organization and rules, but they are among the most despicable people in the history of the world. Are we to assume that the slavers who are typically capturing people in lesser civilized places or buying captured war slaves are going to agree to and follow a set of rules? I just cannot buy it (no pun intended). Each slaver would have a solid fighting force, more than enough to disappear a couple of auditors. Imagine how powerful and able to cross incredible distances a slaver lord would have to be in order to enforce his will on these powerful, secretive and cunning slavers.
Way too long - Hope I made some points. More, I hope you thought of a couple of ideas for things you have or are going to put into your world. Either figure out how your global organizations interact with themselves or agree that they really are just franchises and don’t typically believe the same things.