I was just checking out the Greek Life site for Northwestern and looked at their expansion policy. Northwestern's attitude seems to be that all the chapter houses (which I believe are owned by Northwestern and leased by the groups) should have a chapter in them.
Northwestern Expansion Policy
(Please note it is a pdf)
It appears that the college policy is if a chapter that closes fails to make plans with the school to recolonize within 3 years (unless a longer time is approvved) the house is open for colonization.
Sidenote - Until I looked through the site I never realized how big the chapters were at Northwestern. In terms of stability, only based on chapter numbers, there doesn't appear to be a clear heir for the label of small/struggling chapter. I am just curious about what happened with the chapter that closed because it only colonized about 5 or 6 years ago so if they were really in need of expansion why did the organization never catch on.