I honestly believe that one-world government is very unlikely to happen,
heres why:
"Given the incredible diversity of the world’s people and cultures, it will be difficult to adopt any policy that does not inflict severe harm on at least some groups. The problem of dissident minorities has been difficult to address within individual nation-states. It would be far more severe under a world government." -Ilya Somin

As to a world currency using blockchain, it is a very good idea. Although, it might be difficult to implement without a one-world government.