Nebraska doesnt have a huge in state media market, but a very large collection of rabid fans across the country. There is a reason NU gets a number of national telecasts from ABC. The Big 12 is as big, arguably bigger,of a BBall conference than the Big 10.
In all honesty, if the Big 10 cant get ND, and until the national championship game requires you to win a conference, ND has no interest in joining anyone. Of all the other names that have come up, no one else has the media draw NU does. MU? right....Pitt? they already have Penn State. Anyone they add would have to be a major player in the world of college football. Ohio State and Mich would HAVE to be in the same division. So they need a major power to be in the other division with Wisconsin or Iowa. MU and Pitt dont fit that bill.
Could it happen? Yeah, if the price was right. There is infighting between the north and the south in the big 12, and the north is the loser right now. The question would be, what would Nebraska gain from it. There would have to be a lot more money, and probly garenteed TV games. Will it happen? Probably not. But it would make an interesting debate.