I think there were some predictions/rumors going around that Kougami would end up being either a Greeed-in-hiding (which would definetly explain his drive to gather Cores and/or Cells), or even a/the previous OOO, though both of these are kinda hard to believe since Ankh has had no visible recollection of the guy (yet).
I kinda like the idea of him being a past OOO though, one driven insane by the power he wielded and desperate to regain the power of the Medals by gathering them to him once more.