I'm not sure I see how anyone who buys a property and proceeds to do nothing but sit on it can be considered an investor. From my perspective, they are speculators or worse, slum lords. A true investor will look to find a viable means to improve they're holdings.

At any rate, I'm sure the city will find the ability to pursue any of these guys for whatever areas in which they may be negligent [[taxes, upkeep, etc) to gain title to some of these properties. As it stands, they all stand in the way of the city making any meaningful progress.