What about fully fallen human!Cas. It never occurred to him that he could be possessed or that Crowley or Meg would want to possess him in every way. Would Cas have the fortitude to evict a demon from his body or is he too broken to do anything but let it all happen. And what would a demon want to do with Cas' body or who would they want to do? I'd say Sam if it's Meg and Dean if it's Crowley.