That is still far from happening, it would not be easy and will never happen I guess, bitcoin will never replace the current fiat system as it is fully supported by the government. Bitcoin is successful so I guess we should be already contented with the success that we have and let's focus on a more
realistic development than thinking of replacing the old working system.

Yes agree, then we will not find a gap that is too far away, remember the current wealth of some people in the United States is greater than the foreign countries in Asia and Africa, one world one currency is the best solution.

If it happens,then only the country whose currency is used would flourish and dominate the world economy.Already we could see US dollars dominating the world economy.It has made US richer than any other country.If such one currency idea comes true,then no country would be able to control the currency according to their prevailing market situations.

I think the world will be better if there is only one country and one currency. People will be equal to each other and there will be no racism, no discrimination, no need to trade. But I think that it will never happen because as you say, people only want their own stuff

Having different currencies for country is important. Each country has its own level of growth in its economy. In order to balance the growth of the economy a country should have a central bank that regulates the amount of currencies being circulated in the market. This regulation is being determined by the supply and demand of that specific country. Now, if the world has only once currency. The regulation and adjustment of the currency would be impossible because each country has its own supply and demand of basic and consumer goods. In addition, if only one currency will be used for all the countries, money smuggling will be more easy which will make the economy of a specific country unstable because of inflation or deflation.

Firstly, all the government would have to come to a consensus on it, which is very unlikely. Second, the so called global currency has to be backed by something which is valued by all, which again is unlikely. Say, a country with lots of Gold in reserve would want it to be backed by gold while another country with lots of platinum reserve will want it to be backed by platinum. The only way which perhaps may be acceptable is workforce, which again is hard to determine about the stability of.

The economy of each country is different if all the countries have only one currency only then there is only a division but not the unity and the absence of cooperation of a nation with other nations so that every country is only competing to be the best country not thinking of the undeveloped country.

Wages will be lower

I think that in the world today if we where to all agreeable use one currency that it would either be the yen or China would not be too happy. The whole world's debt would have to converted into one and than so would all of the liquid cash so everyone would have to agree on an exchange rate which everyone couldn't.

I think if this happened it would cause bitcoin prices to go up because its decentralized; however, if the whole world adapts bitcoins something would have to be done about the current high network fees

If it's happen,  I think world economy could be better because no more fiat trading. I also think that no more economical crisis in the world. I am sure that time will happen,  probably we only use gold as money like in the past.