MAC currently has 13 teams because of Temple's bolting. They have a vacancy.
I think the relative values of UNI, MSU, and ISU are close enough such that I think the MAC would take first available. However, I think an eastern team is more in their interests than the midwest. If James Madison or Delaware ever gets their act together and tells the MAC they're almost ready, they'll ignore all 3 of UNI, MSU, and ISU.
If I were ranking them on market, it's probably 1) MIssouri St, 2) Illinois St, 3) Northern Iowa in preference. I think getting into Missouri and having that state open up for recruiting and exposure if slightly more beneficial than downstate Illinois or Iowa, everything else being equal. And if I'm a TV exec, I might care slightly about a MSU or ISU game on a Wednesday night in the middle of the season than I would a game in Iowa.
Based on on-field productivity, it's probably 1) Northern Iowa, 2) Illinois St, 3) Missouri St. Which matters a bit. Not a lot, but a bit. MAC doesn't want a team limping to the barn at 2-10 every year if it can help it. But this ranking doesn't matter as much as the other one.