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.
Btc is not a known currency before, ita just used by few people for them selves as a currency on their group. But now as we can see, it is used now worldwide as a digital currency.