Potential sites include Lucas Oil Stadium in Indianapolis, where the conference currently holds its annual conference championship game. Detroit's Ford Field, Minneapolis' U.S. Bank Stadium and St. Louis' The Dome at America's Center are reportedly also being considered, as is Syracuse's dome. While not going into, Ohio State athletic director Gene Smith confirmed to the Dispatch that the Big Ten is considering those sites "and others."
As far as what those other sites might be, the only other domed stadiums within the Big Ten's geographic footprint are Miller Park in Milwaukee, Northern Iowa's UNI-Dome and Northern Michigan's Superior Dome. It's also possible that the Big Ten would consider stadiums outside its footprint, though that could be a lot more difficult to pull off considering travel.
Maybe you've heard that it gets a little cold in the Midwest during the winter months