Literally anything. We are already seeing a great deal of profit taking with BTC, an explosion of intentional bitcoin forks, the ICO craze slowing down, and non-decentralized altcoins like ripple and Dash increasing in value. It's all unpredictable but if bitcoin goes down from all this pressure alts would go down to. Bitcoin stills acts as the gateway to this economy and if we see that it keeps falling in the ends alts would not remain unaffected.
BTC lost its dominance. it's not currently a gateway. Today eth is the best gateway. Also NEO and BCH become other gateways. Btc has high comissions for entering the market((