If Division 4 requires a conference champion to reach the semifinals who won the championship in a conference championship game, Notre Dame might get some encouragement to join. I'd say the likelihood of this is between 30 and 50%. If October and November scheduling continues to get more difficult due to so many conferences going to 9 conference games, it could also encourage Notre Dame to join.
My wish list for 20 would be UConn, Temple, Texas, Baylor, and Tulane along with Notre Dame. Cincinnati, South Florida, West Virginia, and Texas Tech are also options. Texas has already reached out to the ACC once in 2011, but I don't think they would come alone. They would want friends. They also want to be with Notre Dame, and they are tied to ESPN. We'll have to wait until closer to 2025 for availability of anyone in the Big XII, but if we added 2, then the objective will be 20.