I think there can be huge gains in younger coins, such as Cardano, EOS, ... Coinbase will also introduce more cryptos to its offering (not sure which one but I'm guessing Monero amongst others), which can cause a spike in those coins.

There may well be a drama on one of the exchanges or new taxes, that might cause a temporary drop in enthusiasm somewhere during 2018. Hard to predict those things but there are many exchanges and they are not regulated. Something like a bank run could also happen with an exchange. If they can't return people's coins, this can cause a worldwide panic.

2018 may be the last HUGE year for crypto before it becomes mainstream. Now we are spoiled with 10x gains but at some point, the growth will slow down.

Then there is the effect of the next economic crisis that is bound to happen, probably in 2018. Some claim it will lead to people taking money out of crypto. Personally, I think it will lead people to put more money into crypto to avoid inflation. But I'm not an economist...