If it does happen, will it be like bitcoin or will it still represent gold like how today's money works?
We don't know that for sure because we are all dead on that time but for now there is no reason that every nation will create their own currency just to get rid of their own fiat currency but making their own currency as investment in order to make their fiat currency to become more strong is much possible.
currencies at once? yeah we don't know the condition of the next 100 years but for now it is impossible to implement