Preserving affordable units, whatever that means, does not address a single fundamental feature of the housing market and benefits only a lucky few. Preserved and built is useless. Only built matters.
That said, the tax on vacant land is a fantastic idea and one that has been used to spur development and end Brownfields in several Pennsylvania cities.
The 4 block hole in the ground on 1st Ave between 38 and 42 needs to be built on. Taxes will cure that.