Why does it have to be a single currency? The single-currency reserve is an artifact from the era where settlements required actual wads of cash shipped around. It’s also from an era where most other countries’ currencies were in flux and the dollar was the only non-risky option. This hasn’t been an issue for a few decades now, with euro, yen, pound, franc, etc proving they’re quite stable

With systems like Brazil’s Pix in place (which is supposedly gonna operate across latam countries soon), you can easily settle cross-bank instantly. Countries will then be able to build spot reserves on whatever currencies they need for trade directly, instead of an intermediate “stable” currency

Why can't it be gold again?

It was working pretty well for thousands of years.

The part where countries engage in warlike activities against each other to get their hands on more gold sounds like it would cause economic uncertainty.

We moved off the gold standard in 1933 (domestic) and internationally in 1971 and there haven't been any warlike activities since then, right?

It did not scale.

Hopefully a basket of currencies, no need for one thing.