I think the 'evil substance' the trailer makes referance too is the 'Star Scar Syndrome' I would like to know what Orphans Cloud is living with. And who is the voice of the little girl? Marlene? If Jenova's head was recovered, it would have began to call another reunion to give a rebirth to Sephiroth, as it did in Final Fantasy VII. So, what I think the plot is, that Cloud is trying to stop all of the Sephiroth clones from giving rebirth to him. The man in the wheel chair may be the one trying to co-ordinate all of those things. It is either a) a faint reincarnation of Sephiroth (As was in VII) or Hojo (We never actually saw him die) The room where the wheelchair scene takes place looks a lot like the room Barret was in in Junon, no?