If one day most of countries in the world ban BTC, will someone with a lot of money spend million or even billion of dollars to establish a whole country. In this country, you will exchange with BTC and other crypto. Will you leave your coutry and move to this 'wonderland'?
I think that this concept cannot work during my life time.
First of all you need land for this. And since Earth is taken, you'll have to make the country on another planet. Then this country will have to develop greatly, from services to jobs.
It will also be difficult to have directly Bitcoin, since countries/governments play with inflation depending how good the economy goes.
All in all, it's an idea very hard to implement and it will take time.