I think that we can realistically assume that the total market cap of all cryptocurrencies will exceed 10 trillion USD (in todays dollars) by 2030. Hard to say which coins will make it to the top. Many have already dropped off the charts.
I suspect that the DPoS coins could gain a lot of traction in the next year or two (EOS, Bitshares, Steem). The scaling issue for bitcoin still looms large as a major obstacle to mainstream adoption, but these three don't suffer from that problem.