Mortgage-finance reform doesn’t look to be anywhere on the horizon, but at some point government policymakers will have to figure out what to do with trillions of dollars of Fannie Mae and Freddie Mac MBS if the two government-sponsored enterprises are put out to pasture. In fact, the transition to a new GSE single security that’s scheduled to start in 2018 could become a test run of sorts for the even bigger changes ahead, according to a paper published by the Urban Institute. Crafted by five mortgage-industry veterans, “A More Promising Road to GSE Reform” is centered on the creation of a new government corporation that would replace Fannie and Freddie. The National Mortgage Reinsurance Corp. would issue...