There is nothing to replace it. The reason the US dollar is the global reserve currency now is that the US is the only nation who is exporting enough currency

There is nothing to replace it. The reason the US dollar is the global reserve currency now is that the US is the only nation who is exporting enough currency to allow any two arbitrary counterparties anywhere in the world to get their hands on a common currency. And the only way the US can export that much currency is to run a persistent and structural trade deficit financed by debt. No one else has a big enough economy to throw off that much currency other than maybe China and the EU and neither of those want to dislocate their main economic engines by becoming huge net importers.