For the ETF industry, 2014 went out with a bang. On Dec. 22, total assets in roughly 1,600 U.S. exchange traded funds reportedly topped $2 trillion. That milestone confirmed a stock market rally that saw SPDR S&P 500 (SPY) rise 13.5% last year. But out with the old and in with the new. We asked investment pros to provide their outlook for 2015 and top picks for ETF investing. J.J.