1. AAU school;
2. Not located in current B1G footprint, but located in a state adjacent to current B1G footprint.
If the B1G were to continue to follow that blueprint, that would make UVa the 15th member, and UNC the 16th member. If they go beyond that, Ga Tech becomes the 17th member, and Florida (an AAU member) becomes the 18th member. Florida State is not an AAU member and not likely to become one anytime in the foreseeable future.
Of course, a turf war with the SEC is probably not what the B1G is looking for. I suppose they could try to arrange a Florida State-for-Florida swap on behalf of the SEC, but that could be a tall order as well.