If it ends at Nibelheim, I can see it interchanging with the flashback. Some time anomaly, or Sephiroth's illusion, or the power of memories affecting reality, or whatever crazyness you can think of, making Cloud re-live the incident, while making everyone(including us) confused about when we are, the past or the present? Something pretty crazy, surreal, confusing, and epic, like the end of part one.
Imagine if we have to fight multiple 'Sephiroths'(clones), and then a fully formed Jenova as a final boss(since Jenova's full body was contained in a tank in the Nibel reactor in the past, but since the past and present are mixed together, she/it is still there to fight us). Hell, one of the devs even said they wanted Jenova to help Sephiroth, during the fight in Crisis Core. And we know they tend to use thrown out ideas in later games.
Imagine if we have to fight multiple 'Sephiroths'(clones), and then a fully formed Jenova as a final boss(since Jenova's full body was contained in a tank in the Nibel reactor in the past, but since the past and present are mixed together, she/it is still there to fight us). Hell, one of the devs even said they wanted Jenova to help Sephiroth, during the fight in Crisis Core. And we know they tend to use thrown out ideas in later games.