Could it be possible that in the future there are countries that use bitcoin as the national currency??
This is impossible. You see, national currencies depends on it's economic. And a national currency reflects the nation's culture and history. Bitcoin is digital, you cannot input a face of hero there.
And a national currency should be regularized and controlled by its government.
It is more possible that Bitcoin can be legal and widely used in a country rather than making it a national currency.