Throughout the story, the 'keepers of fate' who are represented as spirits/ghosts show up at pivotal moments in the storyline to prevent changes from happening to the story. The first change has to do with aeris meeting cloud. She doesnt leave immediately after cloud tells her to go, and as a result, the shinra troops find cloud while aerith Is still present. Its at this point that the keepers show up and scare aeris away. Odd, right?
Next scene with the keepers is in the slums, Jessie gets injured so Cloud takes her place for the mission the the next reactor, again the keepers are controlling and bending fate to their will.
After that, the next time we see them is in the church after cloud defeats Reno. Cloud is about to kill reno and the keepers show up and drag him and aeris away to the back room.
1 more example is hojo is about to reveal to cloud and company that he was never in soldier, then he is taken away by the keepers.
At this point, id like you to acknowledge that events are playing out differently to the original and if the keepers were not there to stop said events from transpiring, the story would change dramatically. The puprose of the watchers is to keep the events of the original FFVII in line with the original.
The ending reveals that sephiroth (and to a lesser extent, aerith) are aware of this. Sephiroth knows that he loses in the end if the keepers continue to preserve the events of this ff7 timeline, so he forces cloud and company to fight them.
After the party defeats them, they fight an advent children style sephiroth, even the FINAL battle against Sephiroth in the original is shown, which would confirm that the Remake cant end now in that EXACT same way, the main cast all see a premonition of Aeriths death and the ending of the original FFVII where RED XIII is running through the wilderness, with the actual video clips from the original game itself being used, RED says that its a "glimpse into the future if we fail here today".During the fight once one of the arbitrators are defeated you see the lake in which Aerith is laid to rest yett you can see a stream of light where her body should have been but it clears up almost as if to show that "that future" is now cancelled out,
then you see a remade scene of where Sephiroth drops down to stab Aerith expect Aerith isn't there and its cloud holding out his sword.
Eventually the keepers are defeated and the skies becoming clear, its Insinuated that because they no longer exist, the original timeline and events of FFVII are not going to happen as they did in the original because now the keepers aren't able to bend the narrative to their will anymore and its clear as day that effectively you've defeated the original fate of the main characters and now a new/alternate path awaits. The ff7 you have been playing was never the one you thought it was, and only happened because the keepers tried to keep events the same. Now they are dead and going forward from here means anything can happen, the game ends with clear skies and they say that an UNKNOWN journey awaits them. Essentially they characters have broken free of the contraints of the original FFVII storyline.
I think this was the whole point of letting old players know that anything could happen, it was a bit convoluted at the end but as shown in this post, quite early on you'll get the idea that these new elements being added are going to change the original narrative and by the 3/4 of the game you'll know its not going to end as the original did when your're on the highway out of Midgar.
As for sephiroth and cloud fighting on the edge of creation and sephiroth asking for clouds help, this part is tricky, i see it as a potential redemption arc to fight a greater threat or another version of himself this is one for speculation. Sephiroth should be the main villain so I'm not 100% sure bout this.