Orochimaru, for all his intensive research, has a god complex. Being that Orochimaru continues to research Immortality, so much so that he will chase to no end an Uchiha's body, I question his oblivious nature towards Hidan. Understanding that Kakuzu was indeed mortal, what baffles is why Orochimaru would not do any sort of research on Hidan. Hidan is exactly what Orochimaru would want: Immortal without limitation. Unless Hidan suddenly became dumb from his gaining Immortality, Orochimaru should have been after his ability above all others. Perhaps Hidan wasn't necessarily Immortal but more Vampire-like? He did require blood of others for his 'Ritual'. Still, if you were to eliminate this possibility (highly unlikely, I'd say impossibility really) then what was Orochimaru's reasoning for not studying and/or taking Hidan's body for his own?