The main reason I don't see Clemson being invited is because this expansion is about money and eyeballs (they go hand-in-hand). Clemson doesn't provide anything to the SEC that the conference doesn't already have. They've already got eyeballs in this state (there are more USC alums in the Greenville/Spartanburg area that Clemson). Other untapped markets would bring more eyeballs that aren't already watching SEC (NC schools, west, Nwest expansion?)
And, yes, the Clemson brand is attractive now because they have been doing well, but this stuff goes in waves, and Clemson won't be good forever. Is a 7-5 Clemson squad that is located in the middle of the conference's footprint really that attractive? The more teams they invite, the more the pie has to be split. So, if Clemson is to be invited, they better be bringing more that what they will be taking to make it worth the SEC's time.
Just seems that the purpose of these last additions (OK, Tex & SoCal, UCLA) is to get more eyeballs in different geographical areas. Clemson doesn't do that. Just my too scents