With regard to the composition of the balance sheet, I favor moving eventually to a portfolio of only Treasury securities--that is, without any agency mortgage-backed securities remaining.
It is important to do so in a way that continues to avoid market  16  As described in Board of Governors (2017), the reduction in reinvestment was implemented through a series of gradually rising caps on repayments of principal. Quoted text is from the penultimate bullet point in Board of Governors (2014).