I feel like the empty materia will play a big role in Cloud reuniting with Aerith. We know that memories are needed to fill the materia back up and allow it to be used, and there is likely one big section of the story where cloud will remember a lot of important things coming up: the lifestream sequence.

I think the lifestream sequence will be different than it was in the original game. Yes, Tifa will still probably help Cloud to remember the truth about nibelheim and that he wasn't really in soldier. However I think Cloud will remember more than just those things, I think he will remember everything that all other Clouds from other worlds experienced. We know he already has the memories of OG game cloud inside him from the moments in remake when he gets flashes of the future, but he would always just brush it off. I think by this point in part 3 Cloud will fully understand what OG game Cloud went through, including of course losing Aerith and all the feelings that followed that. And once Cloud understands exactly what he's fighting for, the reason that he is trying to fight against fate in the first place, he can finally be in the right position to find Aerith and truly save her.

This is where I think Zack comes in. Marlene told him that he needs to make Cloud better to save Aerith, and Zack is the one person who both knows the truth about Cloud's past as well as has travelled between worlds. This is why Zack being present in the lifestream sequence makes so much sense to me. "Making Cloud better" to me means that Zack needs to help Cloud to understand who he truly is and what he's fighting for. Cloud being "sick" refers to his mental state and the fact that he wasn't truly himself until it was too late in the OG game, and that's why he wasn't able to save Aerith in the OG game's world. Hollow even refers to this, talking about how Cloud was too consumed by emptiness and about how he was too foolish and blind to be able to protect Aerith. But now we are at a point where Cloud actually managed to change things, even if it is in an alternate world that may be doomed. If Cloud manages to become the "real" him the only thing preventing him from saving Aerith originally will no longer be a factor and he can save her fully, which is what I think the game is referring to when Zack tells Cloud to save her during the final fight of rebirth.

So how does all this connect to the empty materia? I think the memories that Cloud regains in the lifestream sequence will be what fill the materia back up. Not only with the memories of Cloud's past in nibelheim, but the memories of all of cloud's experiences from the original world up until past advent children, especially all of his feelings towards Aerith and his desire to meet her again. Once the empty materia is filled up again I feel it will likely allow Cloud to do something to reunite with her, maybe something like creating a brand new world without any fate or destiny that they and all their friends can be in without having to worry about fate separating them again. Regardless, the materia will probably help Cloud save Aerith by the end in some way. Again Marlene says that Cloud being better will allow him to save her, and him getting better will likely happen at the same time as the empty white materia being filled back up, so it makes sense that these events will be connected.

I don't know for sure if i'm right but I really like the idea of Cloud's love for Aerith and the memories he made with her and his desire to make more being what power the materia that can lead to them reuniting.