The law should be changed so all interest is deductible — or none. The mortgage problems came from people taking second mortgages because it was the only tax deductible form of interest. There are people who bought cars with their home equity lines because it was “a great tax break.”
I advocate making either home rental payments deductible (not 100%, but along a similar percentage glidepath to what mortgagees pay over time), eliminate the mortgage deduction entirely. Anything else is a perverse distortion of the markets.