While Bo Pelini or Turner Gill would of course be excellent choices, there was a reason why Pederson (who Osborne backed for the position as AD) looked outside for coaching. College football is changing and the old NU style will eventually lead the team into mediocraty. While defense is key, and bringing back Pelini to revitalize the blackshirts is top on my list, there are other options for coaches out there.
A dark horse candidate that people may or may not even be looking at is Cincinatti head coach Brian Kelly. In addition to revitalizing that program, he has a long tradition of winning. At Grand Valley State University he won 2 straight Div II championships before moving on to Central Michigan then Cin. He is a winner. Calahan was a tool, he took someone elses team (Gruden) to the superbowl and got hammered there, he failed to gain the respect of anyone in the state with his overly complicated passing offence in a state that is conducive to smashmouth football. Finesse doesnt work when its 10 degrees outside.
That being said, new blood may be good for the Big Red. Kelly is mentioned in other high profile coaching vacancies, namely Michigan (albeit unlikely due to the almost certainty of LSU's coach returning to the Wolverines). If not Kelly, I think it is important not to simply look to an old NU coach to come back. That would be admitting defeat, which I hope even after this season the Big Red is not ready to do just yet.