Like the writers of a long-running soap opera, the producers of exchange-traded funds have covered most of the obvious ground. So in search of new story lines, they've turned to some surprising twists and ever-narrower market slices.

Stock ETFs are baskets of stocks that give investors a way to take advantage of an industry trend or geographic market without betting too much on one company. But with some ETFs, "it's been sliced so thin…you don't have real diversification," argued John Bogle, founder of Vanguard Group.