Let's hope that Gene is correct that the new super conferences that seem inevitable will have 18-20 teams each. That would bode well for ISU.
If they're configured with 16 teams each, it would be hard to make a case that ISU gets in. If they are 4 x 18/20, then I think ISU is in that cut.
16 and ISU would not make sense for the Big 10.
If there really are 4 leagues of 18-20 teams, any solution with ISU not grouped with the Big Ten teams really doesn't make sense.
Hypothetical (and it could go numerous ways):
ND - national add for B10
Missou, a VA team, an Eastern team or two, an NC team or two.
That still leaves them 2-4 teams. They've become gigantic geographically and TV market to the point where they'd prefer just taking another school with BCS facilities that is a quality school and a good geographic fit. Plus other leagues would be expanding and they'd just run out of good options.
4, 16 team leagues, and ISU is most likely one of the 5 current BCS/ND/BYU teams that gets purged (69 teams right now, 64 spots). If not purged, slight chance at being in some Big East/Big 12 coalition survival.
4 18-20 team leagues and ISU is most likely with the Big Ten schools.