I'm not sure you CAN do better than Badelj. Leaving aside the fact that Badelj is a big upgrade on any CM United currently have, who else would feasibly come to a club with no Champions League and no manager as of yet? Rakitic, maybe, but he's not a defensive lynchpin.
Mueller's a pipe dream and a player United really don't need anyway.
One of the Bender brothers would be very well accepted, Fellaini has no future in United so I see no point in keeping him.