re: if the SEC expands to 16 teams.....

Assuming the Gig Ten(+1) takes Nebraska, Mizzou, Syracuse, Pitt and Rutgers as expected if the conference expands to 16, it will pretty much Kill the Big XII.

The SEC would respond by taking Texas, A and M, Oklahoma and Ok State (since OK State is probably tied in with OU)and break up into four divisions, probably like this. Each School would have one main rival that they play every year and play the other teams not in there division in each others stadiums two out of every four years.
It would be similar to what this guy says BIG 16

and the divisions would probably look like this:

West
Texas
Texas A&M
Oklahoma
OK State

Southern
LSU
Arkansas
Ole Miss
MSU

East
Florida
Georgia
Kentucky
USCe

Kinda east
Alabama
Auburn
Tennessee
Vandy