Out of all those Rhule would be my go-to. What he did at Temple and now Baylor has been nothing short of exceptional. Campbell is right up there. I doubt Fickell would do any better than Harbaugh, and would he even want to coach at Michigan? Seems like an OSU guy all the way. Jury is still out on Cristobal. His stint at FIU was lackluster. Seems to be headed in the right direction at Oregon but so was Helfrich at one point. Jury still out on Wilcox. Done well but one 7-win season isn't enough. Shaw won't leave his alma mater and he's regressed since riding Harbaugh's momentum. Stoops is unlikely and thinking Meyer would go to Michigan is laughable.
Again, huge risk when you're going to consistently get 10 wins out of Harbaugh, especially after Michigan suffered through the Rodriguez/Hoke era.