There are relatively few options that can replace the blue block above when the current one expires. The world is very connected and that block ceasing to exist can take down many others with it. Top contenders, for short and long term, happen to be gold, RMB, and Bitcoin. The RMB has the flaw of being run by an authoritarian government that makes even present day America look good. A civilization attempting to use gold for trade in the modern world, would either revert back to the dark ages, or be run entirely on fiat gold, thus defeating the entire purpose and probably collapsing in more scandal than current fiat. Bitcoin, well, I assume you already know what that is, and no, Bitcoin does not automatically lead to fiat Bitcoin.
Let's look at possible scenarios for a random US and Chinese person. We'll go straight to China since what happens there is really the key to everything. A random Chinese person will probably see their standard of living increase with either RMB, or Bitcoin. I think the Chinese population is so large, that gold becoming extremely valuable would have severe distribution issues. Most people would have 0 gold, since gold just doesn't get around that well, but people still need to eat. It would probably bring in an age of serfdom and maybe mass killings of excess population, just like the old days.
RMB reserve currency status brings many unknown variables to the table. Many other countries, such as the US and Japan, would rather go to war than risk having the world being dominated by the Chinese government. This would probably be a world war where the majority of people in China die due to population demographics being very concentrated in small areas, and many Americans and others die as well. It's not really known what happens in this scenario. If Russia joins in, then the majority of people on earth will probably be wiped out.
Bitcoin becoming big in China seems to be one of the few positive things that could actually happen to the country. The average Chinese citizen would probably see an increase in freedom from their government, and accompanying economic freedom or prosperity. The issue of gold scarcity would become a non-issue, since although Bitcoin is a virtual commodity, there will be more than one virtual currency to meet market demand, and avoid extreme, whale manipulation that can occur in single markets.
As for America, the outcomes are similar. With pending fiat doom, most of the gold market will probably be cornered early in the process, if it isn't already, and you'll be in an age of neo-feudalism. Rebooting the fiat system just isn't going to work, people will demand something else. That something will probably be a combination of both virtual currency and gold.