1. Currency flows are invisible to the Fed
2. US sanctions cannot ban mutual trade in other FX
these two are the major ones, so if Brazil wants to trade with China, they don't need to use USD, they can hold each other's FX as some reserve and use it for tradeSomething I think a lot of people miss in this discussion is that with computers and electronic markets dealing with currency conversions is fairly easy. In the past doing everything in USD vastly simplified everything, but today I think countries just trading with whatever currency works for a given market or transaction probably makes the most sense.
There is no other world currency that looks both large and stable enough to replace USD. Chinese Yuan/Renminbi is probably the closest but it's probably viewed by many as too easily manipulated. It's a pure fiat currency run by a single party state with little separation of powers.
If say a Thai company wants to buy cocoa beans from Nigeria, the Nigerian exporter doesn't have any immediate use for Thai baht if they aren't planning to buy something from Thailand. If exports and imports between two countries aren't relatively balanced there won't be a liquid market between their currencies.
You could also create derivative instruments based on baskets of currencies and use those as intermediates.
What does it take to go from bitcoin being measured in US dollars to US dollars being measured in bitcoin.
Just for Bitcoin to no longer be deflationary. Right now it's much too good a store of value to become an effective medium of exchange.