Other than UF or LSU, the majority of the SEC has always wanted FSU in the conference. However, the 200 lb gorillas that are UF & LSU blocked the move.
As for GT, as a UGa Dawg, I wouldn't mind seeing them return. But I have some serious concerns about the standard of their academics relative to the SEC. GT is probably better off in the ACC as long they are pushing academics. (SAT scores requirements were increased not long ago at GT.) If GT gets off the academic kick, and relaxes its restrictions some, GT might do well in the SEC again. Clemson has been the perfect fit for the SEC for a long time, but there has been no interested in bringing Clemson in yet. Memphis is a laugher. The SEC has that one covered and Memphis has bad fan support. I wouldn't mind having Southern Miss, but they would always be on the outside looking in, much like South Carolina, believe it or not, even if they were in the conference. :-[ However, I do think Southern Miss is a good fit in the southern portion of the Big 12, and I'd rather have Southern Miss in the Big 12 than another private school like SMU or TCU.
Which brings me to TCU. Not too sure how TCU would fare in the SEC, but they wouldn't have much power like they do in CUSA now. As much as I like Marshall, I just don't think they're SEC caliber. UCF is interesting though. Replace Marshall w/ Virginia Tech and move the Hokies to #2 on the list, and I think we've got a winner. ;)
Ga Tech does have comparatively higher, admissions standards compared to general state supported institutions. They also have degree programs, exclusive of engineering and business, that are not as competitive and selective as some may think. Regardless, their standards in recruiting football and basketball players are not that lofty. This is common with the exception of the IVY League and some others, mostly not on 1-A level.