ACC schools in the AAU:
Duke
Georgia Tech
Maryland
North Carolina
Pittsburgh (will be ACC)
Virginia
I would guess Maryland as the school that has contacted the Big Ten. I don't see North Carolina looking to leave a league unless they can go with Duke, and the geography is weird, too. The same can be said of Virginia and Virginia Tech (politics are involved there). Georgia Tech doesn't make sense for the Big Ten geographically, either. It could be Pittsburgh, but they aren't technically ACC yet, and they don't add much (Big Ten Network is already on TVs there). Don't forget that Rutgers is AAU as well. A lot of candidates for the Big Ten if they're interested.
Also, if the Big Ten is interested in expansion, let's not forget that Missouri would ultimately love to be in that conference. If their options are Maryland, Pittsburgh, Rutgers, and Missouri, I feel like Mizzou would be high on that list (if not atop it).