If and when it comes time to expand my first realistic choice would be Louisville.
They add a new market, aren't ridiculously far away, just spent $72 million and expanded their football stadium to 55,000, just spent $238 million on a brand new 22,000 seat basketball arena, add a decent football program and a great hoops program. On the downside their enrollment is small and academics aren't that impressive.
To help compare here are some numbers for teams I've seen mentioned here and elsewhere. I did leave out any teams from states we already have since I'm pretty sure any expansion will need to increase our footprint.
Football Stadium Capacity
65,000 - South Florida
64,045 - BYU
62,380 - Memphis
55,000 - Louisville
45,301 - Central Florida
40,094 - New Mexico
35,097 - Cincinnati
34,400 - Colorado State
34,000 - Boise State
Basketball Arena Capacity
22,700 - BYU
22,000 - Louisville
18,119 - Memphis
18,018 - New Mexico
13,176 - Cincinnati
12,820 - Boise State
11,324 - South Florida
10,045 - Central Florida
8,745 - Colorado State
Number of trips to the NCAA tournament
36 - Louisville
25 - BYU
24 - Cincinnati
18 - Memphis
12 - New Mexico
8 - Colorado State
5 - Boise State
4 - Central Florida
2 - South Florida
Enrollment
56,235 - Central Florida
45,713 - South Florida
41,250 - Cincinnati
34,674 - New Mexico
32,955 - BYU
24,875 - Colorado State
23,031 - Memphis
19,993 - Boise State
19,743 - Louisville
Endowment
$832.9 million - Cincinnati
$599.7 million - Louisville
$296.0 million - South Florida
$279.7 million - New Mexico
$180.1 million - Memphis
$148.1 million - Colorado State
$129 million - Boise State
$91.95 million - Central Florida
N/A - BYU
US News Ranking
75 - BYU
124 - Colorado State
156 - Cincinnati
176 - Louisville
179 - Central Florida
183 - South Florida
Tier 2 - Memphis
Tier 2 - New Mexico
Regional - Boise State (51st in the west region)
Two thoughts after looking at all of that.
1. I think I would either expand West with BYU and Colorado State or East with Cincinnati and Louisville.
2. I can see why the Big 12 isn't in a hurry to expand. Maybe a few years down the road some of these schools will have improved some of their flaws.