I'd be shocked if Houston were to get an invite to the Big12. Other than adding one of two needed teams to host a championship game, I don't think there's a value add to the conference.
1. The program isn't that good - finishing in the Top 25 (AP or Coach's) 3 times in the last 25 years and 2 conference championships in the last 25 years. Since 1990 they've finished above .500 11 times and have averaged 6 losses/season.
2. They provide no gains in recruiting territory (Big12 already has Texas, Texas Tech, TCU and Baylor in the conference).
3. Similar to #2, there's no gains in the almighty TV market coverage - should they create a conference network, which they would be stupid not to
I'm no AD, but I think they'd be wise to try to expand their geographic footprint into populated areas (TV network), and areas with good recruiting - if possible. Cincinnati would be an interesting target because you may be able to promote some sort of rivalry with WVU (may be a stretch), get you decent market share and in on the tip of SEC/Midwest recruiting.
I may get lambasted here, but I don't think it would be too crazy for them to consider Northern Illinois as the second team...