I've been paying pretty close attention to this. My prediction is that Oregon and Washington will get into the Big Ten at a reduced payout rate. I don't think Stanford and Cal will make the cut.
1) Oregon and Washington are in the top 4 of Pac-12 attendance, while Stanford and Cal are near the bottom.
2) Athletically Oregon and Washington both make a lot of sense, while Cal and Stanford make less sense. All 4 schools are AAU academically.
3) USC and UCLA are by far the two biggest college football programs in the state of California. The Big Ten doesn't need the San Francisco market, because they already have the entire state locked up.
Conclusion: If I am the Big Ten I add Oregon and Washington at a reduced rate and hold at 18. If Notre Dame ever starts flirting with joining then you can use Notre Dame's historic rival Stanford as leverage and bring them in to go to 20 if necessary.