Unless Jerry steps down or disappears (how likely is that?), there is NO WAY he would come to Dallas. Also we'd have to assume JG would be gone. He is one of the most powerful coaches in the NFL and would demand the same responsibilities wherever he would go. I couldn't see that happening here.
If Reid gets fired there are A LOT of teams who would be willing to fire their coach and hand him the keys so would Dallas be the best situation for him? He might not want to stay in the NFC East and might actually want to go back to the West Coast where his home is (cough cough Chargers).
And lastly if he came here consider that he would probably bow everything up and start over. That means bye bye Romo and Rob Ryan and basically all of the coaches. He hand picks his staff, it would essentially be a clean house. He would probably draft a QB like Geno Smith or Collin Klein (crossing fingers) and migrate the staff he has now.
I just don't see this happening because of the chances of....
Andy Reid getting fired
Jerry handing over his control
Jason Garrett getting fired
Dallas beating more desperate teams in the Andy Reid sweepstakes
One can dream though.