It is reported that BYU is not interested in joining the Big East. Perhaps they would be interested in the Big XII? That conference has lost 4 teams now. Nebraska to the Big Ten; Colorado to the Pac 12; and now Mizzou and TAMU to the SEC. Texas Christian more than replaces Colorado and WVU (reported to be making the move) is a more than suitable replacement for Mizzou. BYU, while not a replacement for the Aggies or Huskers, would be a decent fit. The program is somewhat better than the one in College Station, and they do have a rather large and rabid fan base.
The addition of the University of Houston would be a very good fit for the conference also. Houston can field some very good football teams and, with the right coaching/recruiting staff, could again be a force in roundball. Their baseball program is top notch. Perhaps the Big Twelve should look there to upgrade back to a twelve team league.
Actually two Texas private schools could be considered. Rice and SMU. Rice does not have the fan base in football but the would bring a top notch baseball program along with a decent basketball program to the table. SMU brings decent programs to the table although their is a lack of a large fan base also. The lack of a large fan base at either school may be an issue.
Another program that could be looked at is Texas State. This school is making the transition to Division I and perhaps could be considered. Again the fan base thing has to be considered though.
A decent program to think about might be Louisiana Tech. They would jump at the chance to affiliate with a "major" conference. The facilities are decent and they can, at times, field a very competitive football team. Some recruits, who are considering the SEC, may well take a look at Louisiana Tech (and by extension some other Big Twelve schools) to play ball and further their "education." The addition of Louisiana Tech would give the Big Twelve more of an inroad into SEC territory.
To be considered a top conference the Big Twelve needs to "upgrade" again to twelve teams. While it is impossible to fully replace the Aggies or the Huskers, there are some viable options out there.