Just doing some quick research i have eliminated it down to....
Indiana St.
Creighton
UNI
BU
WSU
There should only be 4, but looking at the schedules it gets a little confusing as I am sure one of the teams had a game scheduled outside of the challenge before it happened (probably a return or something). Each of these teams have played a MWC team the last 3 years. Since UNI and supposedly Indiana St. already have games scheduled for next year, that leaves Creighton, BU and WSU as potential teams that could sit out and depending on the MWC situation next year, it could potentially be all 3.
Not sure how we could leave out WSU and Creighton when they are on an uptick.