If and when cryptocurrency finally becomes an official global currency, how do you think it will affect the global economy and the economy of each countries? Will it help improve the economy better than fiat or will it still need help from fiat?
What do you mean by "official global currency"? If you're asking what will happen when all the countries in the world will recognize it as a currency, than it will probably never happen.
Technically speaking, Bitcoin was always a global currency, it doesn't even know about borders, all you need is Internet connection. So far it didn't dramatically change the world because it wasn't adopted. So, all the potential effects rely on its adoption, and I believe it won't be huge, maybe 10-15% of population at best. Most common uses will be store of value and shadow economy, and while those things are considered as negative by governments, they will tremendously help people.