The Big Ten cares about 3 things:
Athletic prestige, media markets, and academics (in that order).
Athletically speaking there is only ND and the ACC remaining. ND, FSU/Clemson, UNC/Miami end of list. Sure other universities have acceptable programs (Duke/Kansas basketball for example) that can help their cause but the Big Ten does not care, you’ll need to lead off your PowerPoint presentation with something other than your athletic success.
Media markets: Unless they can pull UT/aggy there is no one that will give them a notable Texas presence. Top Non-Texas markets they don’t have: Atlanta, Miami, Phoenix, Boston, San Francisco which is why they’ll have interest in GT, Miami, Arizona St, Boston College (very dark horse), and Stanford/Cal.
Academics: Stanford, Cal, ND, BC, UVA, UNC, Duke, GT, Miami
With this basic understanding it seems obvious that adding ND (the biggest piece of the board) and Stanford (big market, great academics, no encumbrances) is the easiest move in the board to get to 20.
When they go to 24, some combo of UNC, FSU, Miami, UVA, GT.
If they can only land GT/Miami, then they move into their more fringe candidates like Arizona St, Boston College, Duke, Kansas, Colorado, and Utah