There’s nobody standing in their way thus far.
Ultimately, the strength of the dollar comes from a combination of factors that is hard for any rival or group of rivals to match: America's large size and economic and technological strength; the depth of America's financial markets; the attractiveness of America's culture and political ideals; America's economic transparency and rule of law; America's system of mutually beneficial alliances and international financial organizations; and the great power and sophistication of America's military.
The Euro and the EU could not match the power of the dollar and America, so why would a BRICs currency do any better? It won't. Instead, it will be a boutique currency for trade among its supporters. Full access to the world economy will still require dollars though -- even for countries most hostile to the US.