I'm going to play devils advocate here.
What happens to the Texas/Oklahoma game then? That is just as big of a rivalry in all of college football and you are breaking that up. Look, I understand Oklahoma/Nebraska (went to the 1995, 2001 and 2009 games, all wins!

), but it just has not been the same rivalry that Texas/Oklahoma has been lately.
Sure that might be because Texas and Oklahoma play every year, but it just seems that either way you look at it, you will be breaking up a rivalry one way or the other.
Texas-Oklahoma originated in 1900 and we didn't start playing Oklahoma until 1912, even though there were some off-years early on.
I don't think there would be a way to keep us playing Oklahoma each year and have a fair rotation of two other teams in the Big XII South. I like to look at things through our prospective, but the Nebraska-Oklahoma rivalry hasn't been what it used to be. Sure, 2000 and 2001 were great years. But again, those happened when it was us in the north and them in the south. The 90's were all Nebraska though. In the 2000's we went 2-5 against them (yes I know, we had our "worst decade" ever, but still) it is just not what it used to be. I realize there was the 80's, the 70's as well, and those is what a rivalry is based on, but the Big XII killed any chance of being continued. Even with adding a team to the south, and moving Oklahoma North with a North school going somewhere else, is still hard because then you lose the Texas-Oklahoma rivalry. It would be interesting to as Oklahoma fans which rivalry they would rather have though, Texas or Nebraska.
I'd love for it to get back to it's roots. I would hope there would be a way where Texas and Oklahoma could be in the same division, but where we are able to keep our "Rivalry game" with Oklahoma each year and rotate out two other south teams.
I don't think that would work, but either way, you are destroying one rivalry, and lately, the Texas-Oklahoma rivalry is bigger than the Nebraska-Oklahoma rivalry, and that maybe due to seasonal play or Nebraska just being down in the 2000's or separated, but that's how I see it, one rivalry will be lost.