1. Usually already located at a crossroads; you want a mall at a busy junction to attract custom
2. Usually already a local transit hub, since they tend to be a major concentration of jobs
3. Are separated from any pesky residential neighbors by major roads and/or large swathes for parking
4. Have a lot of land in the form of said large swathes of parking
These characteristics actually make them ideal for densification into a more walkable oasis; they already have some mixed uses, and the parking lots are developable without much fuss from neighbors. And you can do the whole thing in phases.
A bunch of malls in the Seattle area are getting redeveloped in this manner.