I think you got it wrong. Cloud did beat him but that was before the original FFVII events, that's why he's supposed to be dead. That's how the story playa out on the original too.This is a sequel, not a remake. Everything had transpired already, or is at least being visioned as such. Cloud flat out says he beat Sephiroth, and Sephiroth says he's well aware. Cloud sees visions of events that transpired in the original game and you even see the ending at some point. He gets tears all of a sudden when he thinks of an event with Aerith, but he can't pinpoint why just yet. The ghosts ensure the old scenario will happen again, which is why they stop things that tend to play out differently (like Cloud not joining the second bombing run, they ensure Jessie is benched). Sephiroth's goal, is ofcourse killing the ghosts. As in the original timeline he loses and he knows it. With the ghosts dead the writers have full liberty. I wouldn't rule out they make some drastic changes to the original plot.