Assuming Kelly is coming back, I hope we would take a long hard look at Strong for DC. I don't know why it didn't work out at Texas - the administration seems messed up in ways we can't even imagine - but he's a good coach. He could probably get a second-tier head job somewhere, but if we paid up and held open the possibility, at least, that he could replace BK when the time comes (maybe even next year!) I could see how that would be more attractive than, say, Purdue.
I think Charlie Strong is a good coach but as I've pointed out before, hiring him as our DC would be problematic.
1) If he is bad, that's a problem.
2) If he is really good, he will just get a HC job at a good program. So it would be a one year rental.
3) There isn't really a scenario where BK gets fired and Strong replaces him. If our defense is good enough to make Strong a candidate, that would very likely mean our team was good enough overall for BK to keep his job. I guess the only scenario would be BK bolting for the NFL or something but I don't see that happening.
Basically the only scenario where Strong makes sense is if we are a playoff contender and he could help get us there. But I don't see a 4-8 team turning things around so drastically that we win 11 games.