As Texas A&M is set to join the SEC in July of 2012, it leaves the door open for several other teams wanting to join the Southeastern Conference.
Texas A&M had a couple of factors that allowed them to join the SEC. The top three reasons are prestige, fiscal factor, and competitiveness. The Aggie football team has had many years of productivity, including 18 conference championships and finishing in the top 25 polls 24 times. The A&M teams also bring a huge television market to the SEC with the Texas market. The Aggies also bring a source of competition to the SEC, meaning they could plausibly place in the top half of the conference.
The several teams considered are West Virginia, Virginia Tech, and Louisville.
West Virginia definitely has prestige within its program. It is the winningest team in FBS football to not win a national championship, as well as have had two undefeated seasons in their program. They also bring a fairly large tv population into the SEC network. The competitiveness of the team is unquestionable after recent history with the team. In the 2005-07 years, West Virginia posted 3 straight 11 win seasons. The one factor that will be an obvious problem is the distance. It is a good 7+ hour bus ride from most current SEC schools, which is a large turnoff.
Virginia Tech is fairly new in the mix of the next team to join the SEC. While the Hokies are prestigious, they still haven't made it over the hump to transition from a consistently good team to a great team. The TV network the Hokies would bring in would be about the same as West Virginia, if not a little bigger. The question with competitiveness from the Hokies is on a year-by-year basis. They are consistently good, but not great. I see them having a hard time getting a trip to Atlanta for at least a few years after joining.
Finally Louisville hasn't shown much interest in joining, if any. Of course Kentucky would throw a fit about it much like Florida not wanting Florida State to join in. Even if Louisville could slip in, they wouldn't be very competitive or bring in new viewers.
The reason the few other teams, such as Missouri, Oklahoma State, Kansas State, etc., aren't mentioned, is because to even out the conference, the SEC needs to bring in a new Eastern Division team. While most fans in the SEC oppose the addition of new teams, there is a feel that these teams would be the leaders in the race of the next member of the SEC.