Nobody is going to agree with me, but I think that, for Nebraska, the "rival" is the team that stands between you and the league championship (like Oklahoma was under Devaney and Osborne. Now, leagues are big (and split into halves) and have a league championship - so if you want to stretch and apply my definition today, it would be the team we expect to battle us for the top of our side of the Big 10 - which is Wisconsin.
However, I think that the whole concept of "rivals" is now kind of dumb. Particularly when you don't get to play some teams again for years, thanks to the large leagues and resulting scheduling issues. There will still be natural rivals if they split a state or a region, like Alabama vs Auburn or Michigan vs Ohio St. but for Nebraska every game is important and we don't NEED a special "enemy". Except maybe Colorado.