This would be great for island worlds.
edit: Well any for that matter.
The idea is simple. Rather than building far far away cities that are tough to manage and at risk of crazy barbarians and such, you start a new child civilization.
It starts with most your acheivments and some other things.
The advantage for you to do this is you can set a all time tax of some sort (until war is declared). You could choose to set it from nothing to ridiculously high forcing them to rebel immediately.
How could this be done?
Well I think if a new feature was added to normal settlers. Rather than build city, it could have build city for <insert civ>. Using a drop menu allowing you to biuld a city for any civ and also a selection being "Start new civ".
When you build a city for a currently active civ nothing extra happens. The city simply becomes whoever you chose (you by default, some "colony" civ you created, or even for some odd reason your rival that you are at war with).
If however you chose "start new civ" you would go through the process of setting certain taxes, or determining what you will give them etc.
Then you could continue to ship settlers to your "colony" and just choose that civ you created to help them along (gotta earn those taxes).

This would really allow for the forming of imperialism and such :D

thats a grand idea. so to my understanding you could tax it really high and then they will rebel but if you keep the taxes moderate then they would be content and stay with you. and you should also be able to use your own settlers to add to the new civ. and loosely manage what they produce.

I dont think you should be able to control any production. They are another civ and should be treated as such in all aspects. You can force a trade for indefinite time in the beginning.
I think the civ should also be forced to like you very high. So it takes alot to push them over.

I dont think you should be able to control any production. They are another civ and should be treated as such in all aspects.

Actually, the control of production was one of the main grievances the American colonies had when they rebelled against England. England basically imposed tariffs on people who tried to make America self-sufficient, even making some items illegal outright. The colonists actually said that they didn't mind revenue meant to generate money for their mother country (remember, they're all Englishmen there), but that they objected to officials that could not represent them imposing regulatory taxes. So, you should have the ability to control production, but your colonists will hate you for it.

Actually, the control of production was one of the main grievances the American colonies had when they rebelled against England. England basically imposed tariffs on people who tried to make America self-sufficient, even making some items illegal outright. The colonists actually said that they didn't mind revenue meant to generate money for their mother country (remember, they're all Englishmen there), but that they objected to officials that could not represent them imposing regulatory taxes. So, you should have the ability to control production, but your colonists will hate you for it.

Well, no, tariffs are a tax on trade. The British were not controlling the production (indeed, until Marx came up with his fatuous ideas, the idea of production being in the hands of anyone other than private citizens was unheard of,) but rather controlling with whom the colonists could trade.

I think it shouldn't be another civ. Only a colony. And they could rebel and become another civ. It would start with ~ the same techs after a civil war.

I like the idea of colonies, and think this idea has potential.

Well I say it being another civ because it would be very simple to impliment. You could easily make it very complicated like in what you are actually taxing them (money, rescources, military even). But that may alter alot of other things.
To keep it simple by just making it so any race can build cities for each other, and adding the "start new civ" option you could do alot without much need to add anything.
This would be by using the same format as diplomatic relations, but forcing it. Also the value for them liking you should be set to high even with taxes (you should really have to tick them off to rebel).
By it ibeing another civ also there is less you have to worry about and you are going to get your tribute, unless you truely are trying to be ruthless.

I prefer something more organic. I would combine a good civil war mechanism with refugees and nomads (described in this thread (http://forums.civfanatics.com/showthread.php?t=122550)) and a system for immigration/emigration to achieve the same goal, as well as several other deeper gameplay elements.