Mt. Gox.
Will it happen again?
Mt. Gox had 75% of trading volume I think.
So if it happens again it cant be an exchange. Has to be
something else which is centralized. The other idea would be the failure of USDT, but I think perhaps dependence on it has waned since earlier in the year? The other idea would be a massive crackdown on ICOs, but I doubt this will come all at once.
ICO crackdown would hit on ETH mostly, I think the initial Ethereum ICO has been the last big one done in BTC. Not a systemical risk.
USDT did not waned at all, more biggest Market Cap since ever, talking 430,000,000 USD today.
The Tether fail would hit on Poloniex, Bittrex, Binance and HitBTC ... those are Altcoin exchange sites mostly. Well Bitfinex, too, of course. The last one could hurt the eastern bitcoin economy except the parts who rely on korean Wong instead of artificial Dollars.