First off, to get a D-Coordinator from another team it would have to either be:
1. With consent from the team since you can't make a lateral move otherwise (and it may cost draft picks).
2. The HC would have to leave/quit/fired and the D-Coordinator not asked to be back.
3. D-Coordinator would have to be fired.
I think our best chance is if Crennel gets fired in Cleveland, which could very well happen (although I don't think he's been given much of a chance there).
I could see him taking the job since he could be the D-Coordinator for a pretty young defense that went to the playoffs this year and has worked for Parcells in the past and the scheme is something he likes to run. And if he succeeds, he's got a decent chance of becoming the next head coach or being a HC for another team in the future.
YAKUZA