ND has played Tennessee the last 2 years. They've played Florida St., Pittsburgh, Ohio St, Nebraska and Boston College in the past as well.
Notre Dame has teams locked in as traditional matchups.
USC - every year since 1926
Navy - every year since 1927
Purdue - every year since 1928
Michigan St. - every year since 1936
Stanford - every year since 1988
Michigan - most years since 1978 beginning in 1887
Pittsburgh - most years since 1930
Air Force - most years since 1969
So, they play 6-8 same teams every year. leaving maybe 3-5 extra games a season - not much room.
They played Army this year because the NCAA allowed a 12th game and Army had an open day.
Future schedules show Duke, San Diego St. etc because of agreements ND had to make in order to move a game a few years ago.