We need more supply of housing AND we need to address housing being seen as speculation, particularly low-occupancy/single family. Don't get me wrong, there will always be a need for a rental market, but we shouldn't have corporations/LLCs buying up single-family/duplex/3-level units en masse and basically setting the market. 
My way too simple idea is a 1+1 - every tax filing individual or household gets a their primary and one additional (vacation, rental, etc) at a favorable tax rate. Beyond that, make it not worth it to just hold properties.