Correct me if I misunderstand... I'm not trying to put words in your mouth... it sounds to me like you think this scheme will cut taxes for rich folks who have expensive houses, not raise taxes for regular people, and yet still be revenue neutral because of increased revenue from empty-house owners paying more... do I have that right (more or less)?
Not at all. I don't really expect it to cut anyone's taxes much, initially. I think it will raise taxes on people sitting on vacant lots or dilapidated houses, motivating them to either fix/build, or sell to someone who will. Eventually, once a better housing stock brings more residents and other economic activity into the city, you can lower the tax rate everyone pays on the land beneath their home because you have other vibrant revenue sources.
One thing I didn't mention that is particular to Baltimore that could make things a little more complicate is that in many cases the owner of the house and owner of the land it sits on are not one-and-the-same.