I think that a most important step is hiring one top-notch defensive secondary coach. I've checked around, and that's what other college teams seem to have (or at least those with top-notch defenses), rather than a safeties coach and a cornerbacks coach. That also solves the problem of Coombs' not getting the job done with cornerbacks.
I get the impression that Meyer wanted Coombs on staff for recruiting and motivational reasons, and carved out "cornerbacks" as a responsibility to give him a position group, and to try to help out Withers in the backfield. I don't think it worked. Leave Coombs as special teams coach (we did see improvement in this area with him in charge). Perhaps give him some other title-responsibilities ("recruiting coordinator" is a title many college teams give to a coach; or perhaps a co-position coach). Let him be a coach "at large" as a motivator.
Although there have seemingly been problems at linebacker of late, Fickell really has shown in the past that he can produce good ones. He has some new materials to work with. He should stay to work with these young players.
Defensive coordinator? Was Fickell in over his head, or were there problems with a scheme negotiated between him and a "co-" that could be ironed out by placing responsibility in one person's hands. Even if he was calling the plays, perhaps he was limited in what he felt comfortable calling by a dysfunctional defensive backfield. If this is what Meyer sees from the inside, then perhaps Fickell staying as sole DC may work. If not, then the new defensive backfield coach need to be someone who can be THE defensive coordinator, even if Fickell retains a "co-" title. He's been a loyal Buckeye.
I think the BIG issue is fixing the defensive backfield, under one first-rate defensive backfield coach. Whoever is then DC then has a chance to succeed.