If the Pac-12, SEC, and ACC go to 16, the Big 10 will have no choice but to expand.
The Big 10 is already, with an equal number of teams as SEC, ACC, and PAc-12, falling behind on the playing field. If the aforementioned three conferences expand, the Big 10 will be the bringing up the rear in market-share of the new "NCAA". The ACC would own the east, the SEC the south, the Pac-12 the West plus Texas. The three conferences collectively would rule the new "NCAA".
ND will never join the Big 10. Besides from emotional reasons stemming from before my time, for some of the same reasons the Big 10 is looking at ND, ND would prefer the new ACC: increase market/brand. By playing in the conference that owns the east cost, plus USC, ND remains national. They also fit better academically in the ACC.