I kinda doubt/hope Eberflus will get a HC position, mostly because of the popular trend to go with offensive minds, but if he does...maybe a hire within? Mike Phair, DL coach, who seems to come from the same sort of philosophy as Ballard. Or maybe the LBer coach, Borgonzi. I think Phair has DC, or at least co-DC experience from his time at Illinois.
It's an interesting question though, since whoever would be the hire would have basically complete control of the defense as opposed to what our OC does under Reich.