There are plenty of threatening international situations which might lead to a banking collapse, but since every bull market "climbs a wall of worry", it isn't a threat unless it happens. The introduction of a new international currency is either never going to happen, or it is going to be needed without much warning. It's needed in the developing world, but there's a long way to go before it topples the banks in the developed world. At the moment, nation-wide index funds would make a perfectly satisfactory currency substitute, like the Spanish bits of eight. They are tested and available in huge amounts, and trusted by everyone who talks about them. They are growing fast and may eventually dominate choices unless something even better comes along. If something better comes along, it is hard to see why we couldn't let the market replace them, as fast as people want to buy them, and in any event, we have tons of gold as an inert reserve. The question is, will they hang around in their present form long enough to be a useful tool?
Of course, no one can answer such a vague question about a vague future for a vague development. But index funds are essentially nothing but bundles of stock certificates which are easy to buy and sell, easy to lock in a vault, and easy to carry. They have real intrinsic value which can change with the economy and whose value can be checked in an instant. If something better comes along, it's hard to see why you couldn't buy it with index funds. It's hard to see why two or more currencies couldn't co-exist, particularly if the co-existence was temporary and brief. It's hard to see why it couldn't be limited to central banks, who support local secondary currencies with instantaneous appraisals of the mark-up premium. On the other hand, it's hard to see why it couldn't be divided into subunits and carried around in everyone's pocket, if that's what is choosen to do. Since we got along with Spanish doubloons for centuries, it can be assumed it will serve the purpose. Since ownership is registered, it's even got one certain improvement: you can lose it or have it stolen, and have a way of getting a replacement.
Resistance from those it would put out of work should be assumed. Just scratch any regulation, and you will find a lobbyist, usually very well funded. But the hard core opposition would be from those who see that the currency has real value if you own it legitimately. Its value as a currency is that it is a real value, a piece of the economy, which you can carry with you anywhere. It therefore upsets the principle of national boundaries established long ago by the Treaty of Westphalia. If you want to defend this fact, you will say you could buy the country that way, by imagining a horde of "tourists" who open their knapsacks and demand what they bought, which is your economy. Some elderly people may remember the French battleship which Charles de Gaulle sent to demand his gold. It wouldn't take very long for that to be disruptive, as the more recent Irish experiment with lowered tariffs also graphically demonstrated with migrant corporations. You will find, even though it is denied, that nations with weak economies don't want to be rescued by richer countries so they will cook their books. A portable ownership of the means of production is not merely portable socialism, it is portable ownership which may be indefensible legally, therefore leads to war. So unless a simple prevention can be devised, sovereignty is the one thing this money won't buy. It would be useful to prevent economic sovereignty as well, and for that we must look to what the European Union devises for its individual component nations, since they don't want to adopt the American one. With the exception of the Civil War, we managed to buy our way out of trouble by having rich states support poor ones, since we are all Americans, right? Until we reach that point, it would be better to have national currencies, based on national economies.
Until someone figures out a solution to that issue, a confederation of national currencies based on index funds is about the best we can offer as a short-term solution. You can gamble on long-term stability, but it's your risk to do so, just as it is today. Short-term is worth something substantial, however. Reducing the cost of trade would almost surely inject several percent of GDP into everybody's economy, net of the cost of doing it.