I don't think that anything should replace Bitcoin. Bitcoin is the leader of crypto and the flagman for the whole industry IMO.
I do not think there is any coin to replace the BTC in the future, I believe the BTC is the king of the Crypto market forever, even if the market is falling, BTC is still a coin occupied. By the end of 2018, BTC prices will rise more than $ 50,000. I believe this will happen.