Well, Bitcoin becoming a world one single currency may sound funny, but in the economy of the present world, there are ups and downs and nothing can be seen to be certain; it is very possible but this may take a longer time to happen.
Some people had emphasized that Bitcoin is digital currency and that it has no physical form, but I think that development and advancement is happening on the daily basis, and obviously, Bitcoin may surpass all these challenges and be position to play more rolls like the the fiat currency.