Nations that are reaching the peak of development, or nations that simply want to detract from the standard fare of infra - improvements - tech - wonders should being having a variety of options to do for their national development. As far as I can see, implementing this feature could theoretically provide the necessities for many of the proposed solutions in this topic that are at this time unfeasible to work into the game such as new resources, new improvements, cities, and even a naval system.
I propose a colony system. For an amount of money ranging from really expensive improvements to the most expensive wonders, a nation can try to invest in establishing a colony somewhere else on the globe under the flag of the players current nation. As I envision it, a colony will be a dangerous, but potentially lucrative risk for medium to large nations. A colony could be incredibly successful and contribute even more power if the parent nation is on top of it's management. A lazy or uninvolved player that opts for a more complex colony wouldn't be able to mange it, and his investment would be lost. Colonies would not be '1-time purchase bonuses' as wonders currently are. You would supply a hearty amount of cash to 'pay each citizen for their crossing', and a minimal number of soldiers to accompany them. These citizens and soldiers would be subtracted from your nation to be put in your colony. The goal of a colony is a long term investment, so don't expect any immediate return.
My proposed features of various types of colonies:
Prerequisites - apply to the parent nation. this is what the parent nation needs in order to support their level of colony.
Costs - the parent nation must have this amount able to send to the colony if wished to develop instantly, or, alternatively, this will be the criteria needed for a growing colony to reach in order to mature
Benefits: gains a nation and colony can make from this level of development
Options: abilities of a colony to develop.
Settlement: $500,000
Prerequisites: Foreign Ministry, 15 tech
Costs: 500+ citizens, 100+ soldiers 100 Infrastructure
Benefits: citizens of settlement taxed up to 23% without happiness penalty
Options: Purchase Infrastructure
Colony: $3,000,000
Prerequisites: Foreign Ministry, Harbor, 100 tech
Costs: 5000+ citizens, 1000+ soldiers, 50+ tanks 1000 infrastructure
Benefits: citizens taxed up to 25% without happiness penalty, +1% population growth, Colonial improvement slots (limited to border walls, schools, clinics, churches, labor camps)
Options: Purchase Infrastructure, Improvements
Township: $10,000,000
Prerequisites: Foreign Ministry, Harbor, 500 tech, 1000 sq miles of land
Costs: 20,000+ citizens, 4000+ soldiers, 100+ tanks, 3000 infrastructure
Benefits: citizens taxed up to 28% without happiness penalty, +1% population growth, Colonial improvement slots (addition of barracks, police headquarters)
Options: Purchase Infrastructure, Improvements
Territory: $50,000,000
Prerequisites: Foreign Ministry, Harbor, 1000 tech, 2,000 sq miles of land
Costs: 50,000+ citizens, 10,000+ soldiers, 500+ tanks, 10,000 infrastructure
Benefits: citizens taxed up to 28% without happiness penalty, +1% population growth, Colonial improvement slots (addition of harbor), New Colonial Resource (to trade with parent nation)
Options: Purchase Infrastructure, Improvements, trade resources with parent nation
City-State: $100,000,000
Prerequisites: ??
Costs: ??
Benefits: ??
Options: ??
When it comes to the colonial resource, by far one of the biggest aspects of this - would create a new resource (one not available to parent nations) that can only be traded with parent nations, or other nation colonies. Only 1 resource would be available.
The Risk: a series of new more randomly occurring events that simply enforce some sort of penalty to population size, soldier counts, or infrastructure that can harshly injure a colony. If at any time, a nation determines their colony is not flourishing, the nation can withdraw all citizens, troops, tanks, and 50% of colonial infrastructure back to parent nation.
Nations can attack other nation colonies, but only with their own colonies, or only with a type of some kind of proposed navy.
Tech does not develop on a colony, and colonies cannot be tech raided.
Please shoot this idea full of holes, and contribute your own insight as to what would make this system work, if at all possible. I believe it would add a whole new depth to the game that would not alienate any particular nation based on size.