These are lots or abandoned buildings that will be cheaper to buy, however, you need to build something new or renovate the abandoned building (can demolish if you don`t like the building itself).
Depending on what you build, you can renovate the public traffic in general in the area, you can build small buildings to large skyscrapers (depends on the size of the lot).
I really like the game and had this ideia here, hope it helps giving you guys some ideas, maybe you find a better way to implement this concept.
Best regards!