Posted by ESPN.com's Adam Rittenberg

After ranking some of the obvious Big Ten rivalries earlier today, I started thinking about matchups that should be rivalries but aren't for one reason or another. This quickly brought me to the Big Ten's list of protected opponents.

Each Big Ten team has two conference opponents it is guaranteed to face every season. For teams like Michigan, the two protected rivals are obvious (Ohio State, Michigan State). For other schools, it becomes a bit tricky. I've always thought the list of protected rivals could be tweaked, though each adjustment would create a new set of problems.

Here is the current list of teams and their protected rivalries:

Illinois: Indiana, Northwestern
Indiana: Illinois, Purdue
Iowa: Minnesota, Wisconsin
Michigan: Michigan State, Ohio State
Michigan State: Michigan, Penn State
Minnesota: Iowa, Wisconsin
Northwestern: Illinois, Purdue
Ohio State: Michigan, Penn State
Penn State: Michigan State, Ohio State
Purdue: Indiana, Northwestern
Wisconsin: Iowa, Minnesota

Now let's look at each team and see if the rivalries could be adjusted.

Illinois -- Indiana is much more of a rival in basketball than football, so there definitely could be a change here. The Northwestern games are usually entertaining, but it's hardly a must-have rivalry. Given the direction the Illini are heading, I'd love to see them play Ohio State every season. The problem is Ohio State would have to give up Penn State, a growing rivalry that easily could have cracked my top 5 list. Adding Iowa is another possibility.

Indiana -- The Purdue game has got to stay. It always was a bigger rivalry in basketball with Bob Knight and Gene Keady on the sidelines. But the football game is becoming a bigger deal, especially after last season's dramatic finish. As stated earlier, Illinois could go, but who gets added to the list?

Iowa -- Two pretty solid rivalries here with Wisconsin and Minnesota. I love the Wisconsin series simply because unlike so many "rivalries," it's so close (Wiscy leads 41-40-2). The Minnesota game could probably go, though there have been some memorable games, including the 1960 game pairing No. 1 vs. No. 3. Illinois or Northwestern could be a possibility here.

Michigan -- No changes needed. The Ohio State game is widely regarded as college football's top rivalry. Though Michigan has had memorable matchups with Wisconsin and holds an impressive winning streak against Penn State, the Michigan State game is turning into an excellent rivalry. Mark Dantonio hasn't downplayed the Michigan game at all, which only underscores the need to keep things as they are.

Michigan State -- The Michigan rivalry has to stay. It means as much or more to Michigan State players than the Notre Dame game, especially since they always have to hear about Michigan. The Penn State game could certainly go. It was cooked up so Big Ten newcomer Penn State could have a rival. The problem is, who would replace the Nittany Lions? Purdue actually might not be a bad choice. Michigan State has a 29-28-3 edge in the all-time series.

Minnesota -- The Wisconsin game has been played forever and probably should continue, though the rivalry's longevity probably trumps its bitterness. As stated above, I'm not totally attached to the Iowa series -- my apologies to the Floyd of Rosedale. Minnesota has played some memorable games against Michigan, though the Wolverines dominate the series (69-24-3).

Northwestern -- Northwestern could easily trade in both of its protected rivalries. The Purdue series hasn't sparked much excitement, and though the Illinois games have been fun, the Wildcats direct their hatred elsewhere. Players freely admit that Iowa has become their top rival, and recent games against Wisconsin have added spice to that series. Northwestern and Wisconsin will meet in the regular-season finale next season, potentially a sign of things to come.

Ohio State -- The Michigan game ain't going nowhere. Penn State has become a better rivalry -- Ohio State leads the series, 12-7 -- but I'm fascinated to see how big the Illinois game becomes. Ron Zook's recruiting has put the Illini closer to Ohio State in terms of talent, and those matchups could be an annual showcase game for the conference.

Penn State -- I like the Ohio State rivalry and feel lukewarm about the Michigan State series. The problem here is geography and history. Penn State really has no natural rival and joined the Big Ten in 1993. Michigan would be a nice replacement for Michigan State, given the two teams' history of success, but that would mean removing Michigan State as a protected game for the Wolverines.

Purdue -- The Indiana game should stay. Purdue's other major rival is Notre Dame, which, by the way, would present some interesting scheduling questions if it ever joined the Big Ten (don't hold your breath). As stated above, Purdue's series with Michigan State is remarkably tight, which always helps for a rivalry. The Spartans could replace Northwestern.

Wisconsin -- Iowa must remain a rival as long as Bret Bielema coaches the Badgers. How many other coaches step on the sideline with the opposing team's logo tattooed on his ankle? It'd be tough to see Minnesota go, especially with Wisconsin catching up in the all-time series (Minny leads 59-50-8). I doubt Badgers fans would get too jazzed about Northwestern, but the teams have played some close games in recent years.

My head hurts. How would you guys rearrange the Big Ten rivalries?