My thought is that if the Big 12 is torn apart, you end up with 4 Power conferences of 16 teams each and only schools from those 4 eligible to play for a National Championship through the playoff system. Notre Dame would have to go all in to the ACC. That would leave a spot in the ACC for 1 more.
Yeah, no brainer Texas is first add if they'll come, but I don't think they will. Outside of Texas, I would think WVU is more attractive, due to geography, old rivalries and being a traditional Eastern Football power, than any currently non P5 conf. teams and is more likely than any other Big 12 team to come to the ACC (meaning Oklahoma schools and Kansas Schools and Iowa St. are more likely to go B1G or SEC, Texas schools to Pac 12 or SEC). Only other possibility is if Penn State wants to come to the ACC, which would absolutely be a much better add for all the same reasons.
Personally, I'd love to have WVU in the ACC so I could travel two hours to watch SU and other ACC games and because I miss the rivalry of WVU and SU and WVU would regain its natural rivalries with Pitt, Va Tech, and SU.