this is one of those "because the west is doing it that way, it can't understand anyone not doing it that way"- thing. Like, the US acts as a world hegemon that sees other countries as its servants and members in common "blocs" as recipients of US orders, so it can't comprehend a system that is not built hat way - which BRICS isn't. For that reason, there's no one country that will, or even wants to, impose its currency on the group.

The common BRICS inter-trade currency will be something new, not used in the member countries (neiuther before nor after - it's not a Euro). It'll be a bookkeeping unit built on commodities to back it, with a central institution that doesn't control the member currencies or monetary politics, but acts solely to bookkeep and to interchange the currencies with the common counting unit used as an intermediary. That's literall all there is to it.