ND will eventually join a conference. They need to play in a conference, compete for a conference championship, etc., due to the CFP. JMO, obviously, and that would change if/when the CFP goes to 8 teams.
If ND chooses to join a FB conference, the ACC is in a good position to recruit them, since ND already plays 5 games a season against the ACC. There would have to be some concessions to allow them to maintain most of their national rivalry games, perhaps also allowing them to keep their relationship with NBC (or at least an independent TV contract), etc.
If (huge IF's, I know) all of that happens, SC may want to consider jumping.