Exchange-traded funds (ETFs) have become very popular among both Wall Street and Main Street investors. Combining aspects of mutual funds and conventional stocks, ETFs are typically viewed as low- cost, low-risk investments. But as we experience more increased volatility and uncertainty, what does the future hold for the ETF ecosystem?  Samara Cohen, managing director and chief investment officer of ETF and Index Investments for BlackRock, shares her thoughts on what the future holds for ETFs and what she believes is the next stage of the ETF evolution.