Since the Delta aircraft have different engines than the ex-AA ones Fed-Ex has, I doubt they'd be interested. It'd be neat if the USAF bought them and converted them to flying tanker/transports, but I think that's unlikely. They'll probably end up as freighters or as the queens of the fleet for a smaller or second-tier airline. (If you could get good prices on them, they'd probably be a sweet deal).
Here's my favorite Delta MD-11 picture.
Click for large version
Photo © Kazutaka Yagi