#13 Notre Dame
Big Ten won't invite Pitt despite how good of a fit it may be. Kansas is iffy but I think Missouri in the Big Ten is the best fit for them as they seem to fit there culturally, academically, and would provide for a natural rival with Nebraska and Illinois.
Kansas is iffy if they get Mizzou, I think they're heading to the SEC and I doubt they'd switch after the fact since they were so publicly embarrassed by the B1G last year (even if it was their own doing). If they don't grab Mizzou then KU has no chance, they'd want KU to help them dominate KC and STL w/o Mizzou they won't be able to do that.
Honestly if it is ND/KU/MU, Pitt is not an unreasonable option, no use in only having 1 school in the North completely isolated from the rest of the conference...but then again that's why I think it'll be Syracuse, Rutgers, Boston College (or UConn).
Thing that needs to be factored in is that the Big Ten is in a fine position now. If the PAc16 came to be, or even an SEC 16, there still isn't anything that says the Big Ten needs to also grow to 16.
The reason being $$$. The Big Ten will only expand if it will make more money for the current members, while retaining some of the identity they prefer (academics being why they passed on Oklahoma last month). So Kansas is a fine academic school, but do they bring in enough money? Likely not.
Missouri seems like a homerun, but even with the PAc-16 about to happen last year, Missouri was left out in favor of Nebraska alone. I still think they are a fine fit, but think things need to change a bit for them to top the list for even #13.
As for Pitt, they are a school I was very familiar with growing up a football fan on the east coast. But I just don't see them ever in the Big Ten. And that reason is that money is the reason the Big Ten would expand. And Pitt is not even the top school in their own city as Penn St. still dominates. If looking to make more money, it means adding new schools that penetrate new markets or in large states with statewide and national audience. It's why the potential for Rutgers in the Big Ten has had more juice than Pitt.
I think there are a lot of fine combos that would make the Big Ten even better. But I do think that Notre Dame is really the factor. No Notre Dame, less need to expand. Meaning, would Missouri, Kansas, Pitt and a 4th make the Big Ten a stronger, richer conference? Maybe not. But in adding Notre Dame (ND would need to start the process), you do open a big door to the northeast. Even if it were just ND/Syracuse/Rutgers and Missouri, it's still 3 northeast schools, so to say, with ND. Swapping Missouri for BC is another option that would work as it really would lock up the region as "Big Ten", even if Uconn, Pitt and WVU ended up in a future ACC.
One thing is for sure, the Big Ten needs to do very little to remain at the top. And if they do make a move, by having 49% of the BTN, the move would have a proven revenue projection model as opposed to having to estimate like the Pac-10 did to get the Pac12 money.