I understand the arguments for ATM but I'm not for Missouri under any circumstances. They aren't even a southern university. They don't fit the SEC profile. Even the B10 didn't want'em. Hell, why not just invite tOSU. Columbus Ohio is closer to more SEC schools than Columbia MO. If most SEC fans won't bother to make the trip to Fayettville what makes anyone think they'd want to drive to Columbia? Since Slive is apparently willing to nuke the conference he might as well go big by adding ATM, UNC, Duke, and either FSU/VT/Clemson and institute 4 N/S/E/W divisions.
ATM - I think enough people have made the case for their inclusion
UNC & Duke - Adds another state and are national names. I'd wager Duke sells more merchandise than MSU, OleMiss, and Vandy combined. In combination with UK they could make the SEC the new Big East in b-ball.
FSU - National brand that's been down a few years but is on the rise. Fits the SEC profile.
Clemson - wouldn't rock the boat or make the conference prohibitively difficult but would be solid mid-level addition. Clemson is an obvious fit culturally.
VT - See Clemson. Adds another state to the conf.