In 1905 there was an attempt to make an entire "colored" (iow: Native American and freed slave) state in the United States. Sadly, Congress did not go for it.

Today the American Indian tribes (aka: Nations) are in fact still in existence, with their own laws and elected governments. They even occasionally have their own election contraversies. Many also have their own tribal property, with their own soverign police forces. They are immune to many state laws, which they tend to take full advantage of, by opening tax-free stores and casinos (to the extreme annoyance of the State governments). They also on occasion conduct their own foriegn policy.

However, all tribal members are also USA citizens, and for the vast majority, quite proudly so. After generations of living in both societies, most tribe members today identify themselves strongly with both nations. So in general, their members don't want to be a separate nation, just their own nation within the USA. Thus it is now really socially impossible for the USA and its Indian nations to ever disentagle themselves from each other. This state of affairs is what Osage anthropoligist Jean Dennison has referred to as "Colonial Entanglement".

If you are interested in what a Native American constitution would look like, Dr. Dennison's book Colonial Entanglement also has several historical Osage Constitutions displayed in their entirety in its appendices. Fascinating reading.

Moving into speculative fiction, Eric Flint wrote a series of books that investigate what might have happened had the Five Civilized tribes managed to set up themselves an actual independent nation, before they got hopelessly "colonially entangled" with the USA. The first few chapters of the first one is viewable for free here. It has some interesting things to say about the difference between organizing a Nation based on self-selected tribes (as the First Nations tended to do) vs. Race (or supposed "blood"), as European societies tended to do.