Something to consider with the ACC - other than Big East outcasts Syracuse, Pittsburgh, and Boston College, the ACC is primarily a southeastern US conference. Many of the ACC schools are also in SEC country. If the SEC expands to 16, it is likely to include more ACC territory --- such as V. Tech. If you have most of those states competing in the ACC and SEC, ND would essentially have to compete for recruits from both the ACC and SEC because they cross territories. They already cross in Florida, Georgia, and South Carolina. I'm sure V Tech would be a tarket if they go to 16.
The three northeast schools aren't even in the same division. Syracuse will be with Boston College, but Pittsburgh is going to be in the other division. That means you would only play 1 or 2 of those schools in a given year and the rest of your league games would be against southern schools. If the ACC had 16 teams, you would have to play at least 9 league games, leaving only 3 non-league games. If you only have 3 non-league games, which rivalries will you end because you have to choose between USC, Stanford, Navy, Michigan, Michigan State and Purdue. If you keep 3 of those 6, you still have no room on the schedule to play Army, Air Force, or any other random team you would like to schedule in any given year.