I think the system could still be made deeper and more complex without resorting to local resources.
For one, have some buildings cost maintenance in gold ... have more ways to get gold/tax populace.
Honestly ... I think POPULATION HAPPINESS would be a strong way to create a deep and fun game with the global system. Sure, its been done a zillion times before, but its *almost* necessary to balance a proper taxation system.
That way you have two sources of iincome ... gold (lots of gold) and people (far less at first).
I'd propose that Taxation be divisable between Gold and Science/Studies ... and that Science can be split into Knowledge and Magic.
(although I guess it'd just turn into a 3-way split anyways ... Gold, Knowledge, and Arcane)
... At first your forced to have no more than 10% magic (per population). You can still build magical buildings through studying the magic tree in "Knowledge" and they will give a few Arcane points ... but they also have nice Percentage boosters that will only be possible once you get Government Civics that increase the percentage of Magic you can gain per population. With the *Best* being maybe 40% max. (but having some penalties, like less gold or pop growth or happiness or something)
Equally, you start with a maximum of 30% science (population based) ... but of course buildings that give Percentages increase not only lost libraries but also the knowledge points gained through population. High end techs allow a Government Civic that allows for maybe 90% tech.
From the beginning you can get 100% gold.
Obviously materials gathered from "resource sites" like a Gold Mine cannot be converted into another resource ... its only the Population Taxes that can be distributed between these three basic "intangibles." Different civics have different effects on not only Taxation maximums, but also various Public Happiness modifiers, overall production modifiers, and troop training speed modifiers (and there can be other benefits and penalties).
So those world resources are quite important at the beginning (and always truly, since *extra* food might be hard to come by).
Also, captured cities should probably have extra unhappiness (and start with 0 influence). Culture/Influence should be handled by faction (similar to civ), so that an occupied city starts out with 99% foreign influence, and slowly reduces foreign influence over time ... meanwhile, the Percentage that is your culture, is the Percentage of Max-Radius that your "conquering influence" controls. Also, your Percentage of Culture/Influence determines how much Unhappiness is in the Conquered City.
You won't be able to capture enemy cities through this Culture/Influence, but such cities will convert more easily after capture.
Certain Military Resources should be blocked from a City until it has a proper building and a trade route with at least one friendly city that produces the resource.
These military resources may be as simple as Metal (requires Smelter and trade-route w/ metal), or equipment/ weapons like Swords (requires Blacksmith and access to metal)
I think such a Building requirement (and trade-route requirement) for basic military resources and certain military equipment ... would make the Global System a lot more doable.
In this model, your not giving even the slightest order to a minion. Instead you have the City "in need" produce a Smelter and a Blacksmith, and you provide a safe trade-route between the City "in need" and a network with access to metal (meaning directly or indirectly roaded to the Metal producing city).
Obviously, if you don't have any natural sources, and you only got the resource through trade, your city in question will need to be somehow connected to the Capital city (just part of the same "network")
Also, I feel that all factions should at least be able to get 2 caravans per city ... while the special few can get 3 or maybe as much as 4 per city. // In late game only of course ... 1 caravan per Faction is fine in early and mid game, at least for those factions not caravan oriented. Also, they need to share the same "caravan techs" so that savvy trading doesn't allow someone to wield more than the "max" caravans per city (like 6 caravans for instance, by having the 4 empire techs and 2 kingdom techs) ... such things that overlap somewhat may be shared (might as well).