Muschamp as D-coordinator at Auburn or A&M could do some serious damage, those teams obviously get top-tier talent, they're just clueless what to do with it on the defensive side of the ball. I could also see him going to South Carolina, thinking he would be the next head coach in waiting there if he does well as a coordinator. Spurrier can't have too many years left, I think he's pushing 70 years old.
He might avoid auburn just because he's already held that position at Auburn once, going back would feel like a bit of a retread. Honestly, I think A&M makes the most sense for him, he's already coached at Texas, so he's familiar with recruiting in the state, but he hasn't been at A&M before, it would be a new experience.
I also wouldn't completely dismiss the possibility of him rejoining Saban at Alabama if someone can lure Kriby Smart away for a HC job this offseasn. Saban + coordinators of Kiffin and Muschamp would be quite the force to be reckoned with. While Muschamp has coached under Saban, that was at LSU, not Alabama, so that would also be a new stop for him, but under a familiar regime.