Bitcoin is already a currency, it does not need to become one. You can buy and sell things with bitcoin and that is what a currency is for. Although now I think people use bitcoin as more of an investment than as a currency but that does not mean that bitcoin has stopped being a currency.
Currently, I believe that the coin is being used as an investment asset only. Till the coin doesn't gain acceptance throughout the financial market, it can't be used as a currency.
Also, the main objective of the coin was to bring forth a decentralised mainstream currency which is not bound to any rules and regulations and the coin will achieve that only when gains some stability. The coin at present is too volatile to be a mainstream currency. The coin has the potential to be one, but until it gets accepted worldwide and becomes stable, it will not be able to achieve its goal.