I doubt the ACC will want to change divisions every year. The fairest thing to to is divide by geography with each division getting a team from a particular region. In a way the divisions are very much like this already. So split Florida (FSU-Miami), split the South (Clemson-GA Tech), split Carolina (Duke-NC State-UNC-Wake), split Virginia (UVA-VA Tech), split the West (Louisville-Pitt), and split the North (BC-SU). Trying to protect as many rivalries as possible, you get:
Atlantic: Miami, Clemson, NC State, Wake, VA Tech, Pitt, SU
Coastal: FSU, GA Tech, UNC, Duke, UVA, Louisville, BC
I think the schools would be happy with this. It is relatively balanced and splits the private schools evenly.