To avoid overquoting...
1. Dropping Minnesota, Indiana and Purdue solely on football is utter blasphemy. Two are founding members and the third has been in for more than a century. They're academic powers, strong in basketball and two of them have had their own glory years in football. Most Big Ten fans wouldn't like the idea obviously including myself.
2. Letting Missouri get away was a huge mistake. They're now a consistent performer athletically, a strong school academically (yes, they are better than NU), and bring in two large metropolitan viewing bases which combined have just as many residents as Boston and probably more, add to that the alumni base twice the size of BC's and a lack of competition from a successful pro franchise. Also, its a school in the part of our country that actually cares about college football, has much better recruiting than Mass and is a manageable road trip for many in the western bits of our footprint.
We share a culture, have geographic continuity and already have a rival for them in Illinois. Also keep in mind that, as far as I have been able to find, the SEC does not charge an exit fee so attracting them would be much easier than stealing BC away from the ACC and its $20 Million exit fee. Add to all of that the fact they actually want to join and Mizzou still makes my list. I want them and Notre Dame for 14.