Nebraska joining, or being asked to join, the Big 10+1 will never happen because:
1. Big 10+1 schools think that academically they are on par with Harvard and they erroneously think Nebraska doesn't measure up.
This actually is one of the more legit reasons why Nebraska won't join the Big 10. Granted, other than Northwestern, none of the Big 10 teams are on par with the Ivy League, but as a whole,
the Big 10 is probably the second most competitive academic FBS conference (after the ACC). The Big XII may be fourth, lacking a really great academic institution and merely being made up of a number of decent to mediocre schools. Being that Nebraska isn't Texas, I don't think the Big 10 would EVER be interested in letting the Huskers join.
Another reason is the Big 10's interest in Syracuse, Rutgers, and Pitt as possible additions, if such an expansion would be made. They make more sense geographically, and to some degree, more sense in terms of academics.
I know many of you often laugh at my rants relating to conferences and academics, but as a student of Higher Education policy, I can assure you that these issues are MUCH more important than you might think.
P.S. For the record, I personally believe the SEC is the WORST academic BCS conference, and one of the lower tiered academic conferences in FBS.