Big 10 played with 11 teams for at least a decade. I understand that people are still mad at Nebraska and I enjoy seeing them twist in the wind a little. But, if it came down to it, I'd be all for letting them back in and having an 11 team conference. Play 9 and you get a year off from 1 team a season. Like it or not, Nebraska would more than hold their own when it came time to negotiate the next round of TV rights. It's not like adding CSU.
Ignoring the context of what happened a decade ago, yes, Nebraska would likely be the best option to add another team to the conference. But that, in and of itself, should dispel any notion of Big 12 expansion. They have a good thing at this point, and adding Nebraska in 2020 is not like adding Penn State in 1990. And for as weak as that star is right now compared to where it once was, it only seems likely to keep getting dimmer.