I don't think a BRICS currency will be successful in replacing the dollar unless the governments of the participating countries force their businesses to use it. The reason why transactions are denominated and settled in US dollars is because of the perceived stability of its value and the openness of the US financial system to international trade. People use the US dollar because they trust the American government to not excessively devalue it and for it to be reliably useful later on.
And yes, I recognise that all of these are under attack by the current US government.