My analysis, potential annual OOC rivalries for each B1G school:
Rutgers:
I would say Syracuse. Most of Rutgers' other frequent past opponents are either no longer D1A (Bowl subdivision) like Colgate, Princeton, etc or are minor teams like Temple, Army, etc. IMHO Syracuse would be a great annual OOC game for the Scarlet Knights. Since 1946 Rutgers is 12-24 in 36 games against the Orange (most were BigEast games) so Syracuse has the upper hand but Rutgers has won one out of three so it isn't completely lopsided and it is
a 4 hour drive which is close enough for students/fans to make the trek. .
Maryland:
I would say Virginia. Virginia isn't Maryland's most frequent opponent since 1946, they are fourth behind NCST, Clemson, and Wake but Charlottesville is a lot closer to College Park than those alternatives as just
a 3 hour drive.
Penn State:
This one is obvious, Pittsburgh. OTOH, I can understand why a lot of Penn State fans might not want Pittsburgh as an annual rival. It might be bad for business generally to treat Pittsburgh as an equal. If not Pitt though, it would be between Syracuse and West Virginia and those don't seem any better.
Ohio State:
There really aren't any logical annual OOC rivals for Ohio State. USC is the most frequent OOC opponent since 1946 but they are on the West Coast and already tied in with Notre Dame. Pitt is the most frequently scheduled opponent (because 8 of the 20 games against USC were Bowls).
Michigan:
This one is obvious, Notre Dame. However, if the Irish and/or Wolverines are uninterested in having this as an annual rivalry then there really aren't any logical annual OOC rivals for Michgian. After ND, Michigan's next most frequent OOC opponents are Navy then four Pac schools.
Michigan State:
This one is obvious, Notre Dame. However, if the Irish and/or Spartans are uninterested in having this as an annual rivalry then there really aren't any logical OOC rivals for Michigan State. After ND, Michigan State's next seven most frequent OOC opponents since 1946 are Marquette, three directional-Michigan schools, and three Pac schools.
Indiana:
This one is obvious, Kentucky.
Purdue:
This one is obvious, Notre Dame. However, if the Irish and/or Boilermakers are uninterested in having this as an annual rivalry then there really aren't any logical OOC rivals for Purdue.
Northwestern:
I don't see ONE obvious OOC rival for the Wildcats but I really like Northwestern's trend of playing other great academic schools from P5 conferences (ie Stanford, DOOK). I'd love to see them set up a rotation where they play Stanford, Duke, Vanderbilt etc.
Illinois:
This one is obvious, Mizzou.
Wisconsin:
There really aren't any logical annual OOC rivals for Wisconsin. Their most frequent OOC opponents since 1946 are Marquette and then a collection of mostly local minor schools and non-local Pac schools.
Iowa:
I'm not a fan of the Iowa/Iowa State annual rivalry because I think it treats ISU as an equal which is not good for the Hawkeyes. If we remove that one though there really aren't any logical replacements except perhaps Mizzou but I already claimed them for Illinois.
Minnesota:
I'd say Washington. Husky Stadium is
a 25 hour drive from TCF Bank Stadium but I think that Minnesota and Washington are the two northernmost P5 schools and other than WSU in Pullman there aren't any P5 schools located in between these two. The Cougars would be an alternative but the Gophers have a fairly limited history with Washington State (6 games since 1946 including one Holiday Bowl).
Nebraska:
This one is obvious, Oklahoma. Technically the Cornhuskers have more games since WWII against Mizzou, Kansas, ISU, KSU, and Colorado but without the need to look it up I'm confident that those series are substantially more lopsided than the Cornhuskers' series against Oklahoma which also has some VERY memorable past games. If Oklahoma isn't possible then I would consider Colorado before Nebraska's four most frequent opponents since WWII (Mizzou, ISU, and the two Kansas schools).