A few points...
TCU & WVU got invites to the B12 because the B12 was down to 8 teams and needed content for its cable deal in order to keep the contract valid. TCU and WVU both had history, branding and success on their side. It was not just one factor or another, but a combination of factors. Had the B12 not been in dire need of teams, probably neither school is in the B12 today and are competing in the AAC.
TV market size is not as big of a factor to the B12 as some would like to believe. It is a huge factor to the B10 and SEC because they have their conference networks to sell. They can demand their channels be carried in basic packages for premium prices in large metro areas if they have a team with a large enough fan following in that area to demand the channel be carried by basic cable. The viewers in the area are more willing to pay for the channel to air in basic packages than are fans in areas not associated with the school. That means more money per subscriber for the conference.
For the B12, until the Longhorn Network goes away, bringing in schools just because they have a large market is moot, because the B12 can't take true advantage of them without a network to force sell to cable providers. That was the appeal of Rutgers and Maryland to the B10.
UCF is probably more appealing to the B10 than the B12 if market size is the sole determining factor, but it isn't.
A major factor working against UCF is TV appeal due to branding.
BYU has national appeal because their brand has been around for some time. Boise has branding, but theirs was more of an overnight success type of branding. Their upset of Oklahoma made them the talk of the nation, because they beat a blue blood school on a national stage. Prior to their win, no non-BCS school had won a BCS bowl, and there was anticipation as to when it would actually happen. The fact that they did it first was a huge boost to their attempts at building a national brand.
And do not kid yourself. Winning matters, but more than that, winning consistently matters even more. Boise has been a fixture in the top 25, even top 10, for nearly 15 years. If Boise wasn't winning big and doing it consistently, they would just be another Wyoming. Boise has also beaten schools since then on a national stage, such as VT, Oregon, and Georgia. Their consistent winning has gotten them into 4 BCS bowls, which is huge exposure, and they have won 3 of those.
Boise has also proven their success is not tied to a specific coach. They have won big with several coaches over the years. They have had a few 8-win seasons thrown in, but they always follow it up with a bounce back season the next year.
They also dared to be different and have that blue turf. You see a blue turf on TV, you know you are watching Boise. It is a signature that differentiates them from other programs.
The problem for UCF is that your BCS win came against an upstart. Yes, Baylor has been around a long time, but they have also had a reputation for being a loser for a long time. The year you beat them, the old guard fans were almost waiting to pounce on them to say, "See, we knew you were not for real." Your win actually hurt Baylor more at the time than it helped you. You are unfortunately fighting perception. The old guard doesn't want new comers rolling in and taking any of "their" money.
What made it worse for UCF is you followed up your best season ever by taking a flaming nose dive just two years later. It brings into question whether you can sustain success, or whether your historic season was a flash in the pan where you lucked out by finding a diamond in the rough like Bortles. It isn't the fan's fault, but unfortunately you get to be the ones to pay the price.
I can tell you that Memphis is not on the B12 radar. That is more of a generation by the media. The B12 is smart enough to know their success is directly tied to Fuente, who just left by the way. They have a history of losing. They had a good thing going with Tommy West, it seemed, but that was proven to be more about what DeAngelo Williams did than what West did. They have geography in their favor and a top 50 market. Their basketball team is typically pretty good. But they don't draw flys to their football games, and they are behind the NFL, NBA, Volunteers, Commodores, Nashville, Graceland and the Smoky Mountains as far as public interest in their own state.
Cincinnati is probably closer to an invite than Memphis because they are competitive consistently in both sports, they are in a fertile recruiting area, they have geography on their side and they have a large tv market, but we have established already market will not be the deciding factor. Still, if the B12 wanted Cincy, they would already have them.
Houston is never going to be in the B12 unless all 4 other Texas schools leave the conference first. End of story.
The preference for the B12 is not to expand, unless schools like Notre Dame, Florida State and Clemson become available, which is not likely at all.
If I had to place bets on who the B12 might expand with (assuming the B12 HAS to expand) I would say the top 2 candidates are BYU and Boise, in that order.
The second option out there that has a small rumor following is BYU, Army, Navy, and Air Force to get to 14. All 4 have national following and branding. The B12 doesn't need any other power schools. 6 different B12 schools were ranked at one point or another this year, and that does not include Texas who will eventually get back to winning. Adding those 4 schools get the conference to 14, on par with the SEC, ACC, and B12. All 4 have history and tradition. All 4 are TV friendly. They give WVU some close neighbors within the conference in Army and Navy. Air Force puts the B12 back in Colorado, and BYU is self explanatory. Plus, Annapolis and West Point are historic college football venues.
As far as measurables go, UCF has what it takes. You have the size. You have the market. You have the recruiting grounds. You have a solid and growing fan base. The only thing holding you back is winning against some big name programs on a national stage, which has to do with proper scheduling, and most importantly, winning consistently over a several year period of time. Boise State only started playing football in 1968. If they can do it with all the disadvantages they have, then you can do it, and probably do it better.
Either way, good luck.