Why Duke and North Carolina makes the most sense
1. Brings in new TV markets. The North Carolina TV markets are not huge, but where else can the SEC go? As I mentioned earlier, they already have a strong TV footprint in South Carolina and Florida.
2. It adds prestige to the conference. North Carolina is the flagship school of the state and one of the better public universities in the country. Duke's reputation is well known and would finally give Vanderbilt a fellow private school to commiserate with
3. They wouldn't threaten any football powers. The SEC doesn't need help on the football front. Adding Texas will already shift some of the power. Duke and North Carolina would not. They might pop up and have nice seasons on occasion, but neither is going to become a perennial power.
4. It would make the SEC the premier basketball conference. Duke, North Carolina and Kentucky would be a hard triumvirate to top. Throw in the Texas and a few hot middle of the road SEC teams and you would easily have the most talked about hoops season every year.
5. It would kill the ACC. The remaining ACC schools would probably form some sort of super conference with the Big East leftovers, but there wouldn't be a hoops matchup to carry what has traditionally been a basketball conference.
Would Duke and Carolina leave their comfort zone? They are both so attached to the ACC. Neither school liked giving up power when the ACC expanded. If they were to leave for the SEC, they would see their voices diminished that much more. But money talks. If the SEC came to them with the Godfather offer, they would certainly listen.