It depends on how much 'room' the act would need to slip past Sephiroth's current control. Zack manifesting for a moment of quick communication with Cloud is a lot easier and quicker to pull off compared to Aerith releasing her Rain or dissapating the entire Negative Lifestream. It's like, say, Sephiroth left the front door to a house cracked a few inches. Now, a kitten could manage to get through, but a Great Dane would still not have enough of an avenue to do so.
If anything, I would think the door would be cracked a lot less given the fact the entire Negative Lifestream is a manifestation of Sephiroth's will, compared to the Lifestream just being a manifestation of the planet's will. And that's a whole lot of supposition right there, when nothing at all even states Zack to be in that situation in the first place.
Did you watch the ending of AC/C? Or Crisis Core? Physical bodies are shown dissolving into Lifestream and leaving nothing behind. Kadaj, Loz, Yazoo, Lazard, numerous Genesis Copies and an Angeal Copy did. It's implied that Zack did so as well, because at the end of CC there is a shot of the cliff he died on and there isn't any body. Where did his body and cellular matter go, if not the Lifestream? A pack of Kalm Fangs certainly didn't come along and scavange it, bones and all.
I also took the ending's last view of the scene as symbolic, considering Zack isn't quite the same as the bodies of the remnants of Sephiroth, or fully mutated Genesis or Angeal copies. Considering they were so far into their degradation and transformation into monsters, and the remnants didn't really have bodies to begin with...it makes sense for them to just vanish into lifestream. Zack however, isn't like them at all. A body would be left behind, as shown in the original FFVII.
Yea, okay, but he has to have something that makes him able to have a sway on those people in the first place. Geostigma is a manifestation of the body's rejection of Jenova cells. Sephiroth 'branding them with his will' is just a fancy way of saying he told the Jenova cells to cause a ruckus instead of lying dormant and benign.
Yeah, that would be his will.
Perhaps those that had died before he did simply wasn't a large enough number for him to be satisfied. Which makes perfect sense, since he didn't have enough Negative Lifestream at the end of AC/C to choke the Planet effectively either, even though it had been two years of a world-wide epidemic killing people and fueling him.
All he had to do was keep waiting really. If Sephiroth's will was capable of absorbing spirits of the dead who had Jenova cells already in them, he'd have had no trouble at all enacting such a plan even in FFVII.
How couldn't he control the cells? He was able to control them on the living plane from already in the Lifestream well enough.
Well if Lifestream Black is any indicator on how Geostigma came to be, Sephiroth had to shed part of his memory and soul within the lifestream and then go around the planet, using his will to brand people with the mark. He had to in essence spread himself the entire lifestream.
See above in my response to Tres. Hojo more than likely didn't think Cloud was a success until meeting him again at the Northern Crater.[/spoiler]
I remember those memos but they seem to contradict FFVII too... how is Cloud being a failure by acting just like all the other guys? Being non-responsive zombies? I don't get that.