I've been seeing people speculating that Sephiroth is going to be using the whispers that he absorbed going forward in Rebirth. I have a different interpretation of the ending of Remake and it also explains why we fought Sephiroth at the end.
Cloud and party fight the Harbinger/ Viridi / Croceo / Rubrum. Afterwords we find ourselves in what appears to be the lifestream
We have weaken them to the point that Sephiroth absorbs them in that CGI scene. This most of us agree upon.
Great! Sephiroth has absorbed the whispers so he'll be using their powers in Rebirth, right?
Well then, why did we fight him?
I think Sephiroth that comes down to fight us right afterwords is a lifestream manifestation of himself that has now embodied the whispers inside of himself. His intention is for us to fight him and win and fully eradicate the whispers from the lifestream.
As we defeat this manifestation of Sephiroth, whispers fly out of him. He throws them at the party....
We see slow motion shots of Red XIII flipping and swiping and Tifa punching etc. As they attack each whispers they disappear because they are almost completely at the point of being erased from their existence within the lifestream right before Cloud lands the final blow....
As Cloud comes down with his sword overhead, the moment Cloud hits Sephiroth whispers fly out of him.
This is because the Sephiroth that we were fighting this whole time was a lifestream manifestation of himself that had embodied the last of the whispers. It's similar to how when we finally beat Jenova in the fight earlier, they transform back into what it really was all along which was a Jenova clone manifestation as Jenova.
So after Cloud lands this hit and all the whispers shoot out of this Seph manifestation, they all get sucked into a single point and explode into a white light. This is the final eradication of them inside the lifestream which is exactly what Sephiroth wanted the whole time and the whole point of us fighting him.
Cloud gets instantly transported to the edge of creation where time has completely stopped. So from the party watching Cloud land this final hit, they don't wait for Cloud to be done at the Edge of Creation because they don't experience any time go by. They most likely all find themselves at the outskirts of Midgar where Barret is like "I though you beat him?"
So switch over to Zack, the whisper Dome explodes in white light, this is correlating directly to us exploding the whispers within the lifestream. I believe it's implied that these explosion are basically happening at the same time relative to each other. Remember the edge of creation scene time has stopped as it's in the blink of an instance to anything else that is going on.
Sephiroth just wanted the whispers gone from existence so he can defy Destiny, I don't think he need to use them, he just need them to be gone.
I'm sure the whispers will show up in some way in Rebirth as the devs have confirmed that, but we don't know exactly in what way that will be yet.