Since this subject somehow got bumped and this is something I contemplate from time to time I'll toss my two cents in again.
If there is a expansion I think it has to center around expanding the reach of the conference and increasing the SEC fanbase, maintaining a good reputation while making the SEC slightly less self destructive. The key is popular/competitive schools with good attendance or geographic locations that can pull their own weight without making the SEC impossible.
Currently there are 4 non-conference games, 8 in conference games 5 of which are the division. If you expand the conference to 16 teams, you go with one static out of division game and the rest are in division. This can preserve rivalary games (such as UT) while limiting the chance of rematches in the SEC championship games.
The right additions and you could add in-state rivals, add more automatic bowl games, expand regional television broadcasts, prevent SEC rematches and perhaps even open up another at-large BCS game. What I wouldn't want to see is a team with very poor attendance or a third team from a particular state. They need to pull their own weight and three is a crowd... A major key is that you have teams people will care about and games people will care about even if there is a off year. So, rivalries and good attendance are important.
Based on these ideas here are some teams I'd want considered:
Clemson: Clearly the top candidate in my mind, good but not great. Great attendance, in-state rivalry for South Carolina.
Louisville: They show decent support for their football team, but aren't too good. Would bring in good basketball revenue and has a in-state rival.
West Virginia: Better traditionally than one might assume, litterally blue collar. Good attendance.
South Florida: Competitive, lot of students, in-state rival. Big potential TV viewership.
North Carolina: The SEC would gain a lot from expanding into North Carolina. Their basketball team would be a big plus.
Marshall: Only a consideration if you add West Virginia.
There are quite a few other teams that could fit. NC State, East Carolina, Georgia Tech, Florida State, etc... I think you'd want a couple perfect fits to build around then you'd see what else would fit.