The B1G sounds like it is very much sticking to its requirement that league members also be members of the Association of American Universities. Only Ga Tech, Duke, UNC and UVA are members of the AAU, so I doubt a full-scale ACC-B1G merger is in the offing. Largely based on grad school rankings and research activities. Billions of dollars worth of research grants. Makes football $$$ look like crumbs, so it seems unlikely that B1G presidents are going to give up that gravy train to let non-AAU schools in the door.
From the PAC 12, Arizona, Colorado, Washington, USC, Oregon, UCLA and Stanford are all AAU members. Some kind of merger there would be more likely. Kansas and Iowa State are as well. They don't bring much to the table athletically, but they may very well end up in the B1G after the Big 12 implosion.