One of the biggest challenges that we will encounter in Bitcoin is the Scalability problem which means there is only limited amount of information that can be contained or its capacity. I'm sure that there will be more people who will going to join the campaign which will result to a possible delay during processing or trading. So how can we provide a solution on this or how to ensure that all the trading done were saved once delayed before bitcoin achieves mainstream adoption?
I think this issue always exist. But i think the biggest challenges in cryptocurrency including bitcoin is regulation from government. When crypto going bigger and government wont issuing regulation, market will unstable because investor will not confident with their investment. Regulation like or not is needed