1) Missouri- traditionally strong basketball and somewhat decent right now in the pigskin. It would give Arkansas an actual rival. Downside... it's a long way to Columbia.
2) Louisville- basketball power and would give Kentucky the conference rival they so desperately want. I'm aware of the rivalry already, but being in the same conference would jack it up big time. Football... they are suped up Ole Miss... good enough for me. I don't see the downside with them.
3) Kansas- they seem to be the one left out of the expansion party and without a date. Bring them into the SEC and we instantly become relevant in the CBB world. Kansas would make the SEC West competitive. Downside is it a long way to Lawrence... unless you fly.
4) Florida State- they have all the tools to be a good basketball program... just haven't found the right fit. While their football has taken a nosedive I suspect they'll be back after they fahr Jimbo Fisher. The downside... Tallahassee is a dump. Upside- it's actually in the southeast.
This is a basketball blog so I'm obviously biased toward making the SEC a better basketball conference. Saying that, the SEC is the premiere CFB conference and that won't change if the PAC 10 and Big 10 expand.