Big Ten has made note that the only expansion they would do would be within their current footprint, or bordering their current footprint. Would have to be a University that was AAU member and most likely Carnegie designation: RU/VH. Success on the playing field would not have to be "world-beaters".
Given that they now have the Big Ten network, market demographics would be a big factor in the decision.
Pitt is AAU, RU/VH, has great facilities, is in a big market, but doesn't add anything that Penn State doesn't already deliver. Would be hard to break-up the "Backyard Brawl".
Syracuse is AAU member, but only RU/H. Market is mediocre, private school not public (lacks "state" drawing power), facilities are mediocre.
Rutgers is AAU & RU/VH, biggest market available. facilities are solid and getting better. Solid in all sports, would make the Big Ten the dominant conference in the northeast as Penn State and Rutgers would be a bigger "pair" than BC and UConn.
Missouri is AAU & RU/VH. two big TV markets in KC and StL. Facilities are pretty good. Solid in all sports. Already has rivalry with Illinois, but bigger one with Kansas.
Nebraska is AAU & RU/VH. TV market is small. Facilities would be at the top with Wisky, Mich, tOSu, Penn St. below average in basketball, historically elite football program. Could balance football power between an East/West alignment; rivalry with OU has already been "broken" by Big XII.
Iowa State is AAU & RU/VH. TV market is minimal, as Iowa already is the big school in the state. Facilities would be below average. Decent in hoops, below average in football. Already plays rival Iowa in non-conf matchup.
Maryland is AAU & RU/VH. TV market is really big. Facilities are mediocre but would be below-average in Big Ten. No real "rivalries" with Big Ten teams, and is a founding ACC member. Up and down football, but consistent basketball team.
None of the other programs that fit the footprint have the academic credentials that the Big Ten seems to "require", not even Notre Dame. But, of course, the Irish have a long-standing invite to join.
If not ND, then I could see Maryland, Rutgers, or Mizzou as their top 3 choices. Rutgers is the "near-Ivy" type school with long-standing traditions, a HUGE TV market, and wouldn't "threaten" the current top teams. Maryland is close to on-par, but Rutgers just seems to make more sense to me.