Looking at the map of BRICS, it's missing representation from South America, the Middle East, Southeast Asia, and Africa.

By economic size, Argentina (2nd largest in South America behind Brazil), Saudi Arabia (largest in the Middle East), Iran (5th largest in the Middle East), Indonesia (largest in Southeast Asia), and Nigeria (largest in African) make the most sense for inclusion. It provides a strong counterbalance to the North American-European axis of influence that's dominated global affairs since WW2.

Iran's inclusion forms a necessary counterbalance against Saudi Arabia and can be used to further diffuse the tense relationship between the two countries. The rest are fairly self-explanatory.