If the Big XII was to vote Baylor out, and I'm not sure that will get serious consideration...obviously Arkansas would be the logical "dream" replacement and a natural rival for UT & OU, not to mention Missou. But...Arkansas fits pretty well in the SEC, too and revenue sharing in the Big XII falls short of what Arkansas gets from the SEC every year.
I still think the NCAA decision on whether to allow the ACC to play a championship game with only 11 teams will be the key to whether the Big XII even considers ousting Baylor.
If Baylor dropped out on their own and the Big XII needs a "southern" division replacement, I kind of like UTEP as a candidate with more similarities to other Big XII schools, not to mention the Sun Bowl. Houston would be another possibility. I just don't see another private school.