Having one currency is possible if we are united and acting as one nation but if there is a lot of countries that are separated to each other then expecting to use a one currency only is very impossible. One of the requirement also for that one currency is, it is easy to use in transacting not only by physical money but also virtually so there will be no problem in transactions.

This is very hard for all government to use one currency for the whole countries, technically it is impossible so that I think there is no need to make any comment about this from my point of view, there is big difference among their economy some countries are poorest.

Global currency would imply that you can use the American Dollar anywhere without exchanging it, which is far from the case. Of course there's a couple of countries with weak currencies where people take USD directly, but there's still... the rest of the world.

I wonder how will they distribute the money. Is it going to be a digital one? How are we going to convert our current currency to that new currency? Is the government free to continuously print it like the way they do in the current currency? A lot of confusion might happen but one thing is for sure, it will create chaos. Nations will be fighting over this new currency because there will be millionaires, maybe even a whole country, holding the currency to put a lack of supply of the currency to make its value go higher. There will be hardcore holding going on.

Humans aren't the type of people that will attain peace just because we all use the same piece of paper as a payment medium. I absolutely see no point on throwing out all the existing paper money and then reprinting one currency that's enough for the whole world.

Possibly a war never happens anymore because now people has started to realize the value of human to some extent. The one currency system will not be effective as there will be discriminated growth happening between Countries.

in very beginning it look like difficult, but with the passage of time everything will become ok. although implementation is not is not so easy it s time consuming but we need to take the step and start working and only then it can be possible. otherwise only planning is just to waste the time. 

It seems that you think too shallow. Not just at the beginning. This will continue, if it can be realized so badly that there will be so much poverty in the world, dikarnakan can not keep up with monetary developments.

I do not know what makes someone want to make the currency into one? If there is only one currency then the balance will be destroyed. Imagine what will happen to the market when there is only one currency?
I think there's no need to think about it. We can use dollars anywhere. Without losing any other currency

nothing good will happened if the scenario applied ,
one currency indeed will only causing some chaotics ,
it's just like a random idea which should not considered at all as a serious one ,
the world will just turn crazy , and it's impossible to happened i guess.