The natural fit for TBS would be to air SEC or ACC football. However, both conferences have their cable rights with ESPN. Neither the Pac-10 nor the Big 12 have an ESPN cable package, and apparently whatever deals these conferences have with FSN do not call for cable exclusivity. TBS must have been a good fit for the conferences, since the channel is available everywhere in the country on basic cable, which is not necessarily the case with the hodgepodge of FSN regional sports networks.
SEC sells football broadcast rights to CBS, ESPN, and Lincoln Financial Sports (syndicator who produces games and sells the rights to mostly over-the-air stations in the South).
ACC sells football broadcast rights to ABC (where games are labeled "ESPN on ABC"), ESPN, and Lincoln Financial Sports, in a joint venture with Raycom.
There's no room for TBS to get an additional game from either of those conferences to broadcast, unless they wanted to be relegated to broadcasting Vanderbilt (SEC) and Duke (ACC) football games every week. That's likely to be the only offerings left after all the aforementioned broadcast partners selected their games for the week.