Marlene's Foresight
Marlene's being able to see into the future like Aerith and Nanaki. However, they all only see the OG Final Fantasy 7 version's future.
She did not want Aerith to wake up from her coma because Cloud is still sick either mako poisoning or his mental state. She did not elaborate his condition. Zack assumed Cloud's mako poisoning needed to be cured. Aerith were to wake up without Cloud being cured. She is going to die by the hands of a scary man.
She talked about how Cloud tried to save her but failed in the end.
In the OG, Cloud tried to kill Aerith through Sephiroth's manipulation but did not do it. Cloud stood there watching Aerith pray without realizing Sephiroth was going down to kill her. She probably does not know every single detail of her dream about that event. She only knows about a scary man killing Aerith while Cloud was in the area. If Cloud would have known the future or noticed Sephiroth in time, he could have saved her.
Marlene wanted to change the future by having Zack to cure Cloud's sickness.
To the outside looking in, people like Zack would assume about the way she was talking. She might have had a bad dream. However, she convinced him by describing the scary by his hair appearance.
It is all about assumption from Marlene's part to have Zack cure Cloud's illness in order to save Aerith. Marlene only had the knowledge of the OG timeline not the new timeline. She does not know about what happened at the end of Remake. It created a new timeline where Zack and Biggs survived. The new timeline known as Terrior Timeline/Universe.
The main timeline is the Beagle Timeline/Universe where the main gang is currently.
In the Beagle Timeline/Univers, Cloud did deflict Sephiroth's sword without having Zack to help. It did create a new timeline by showing a rainbow effect like when Zack would make different choices to creating new timelines. One timeline where Aerith lived and the other timeline where she died.
Marlene's statement about Zack having to save Cloud in order to save Aerith was not true. Aerith had already been saved by Cloud without Zack's help. Aerith does not need to be saved again. Zack told Cloud to save Aerith even though Cloud already saved her. It was more on Zack's end on how timelines are different from one another. He had been trying to play catch up with the whole story since the beginning of Rebirth. Like how people known Cloud being a merc and running around Midgar even though Zack was with Cloud. Zack's most known qoute from Final Fantasy 7.
Marlene told Zack about Aerith's feelings towards Cloud. Whatever Marlene viewed it as true or over exaggeration because she is 4 years old, after all. It made Zack's heart more in despair. Zack's sole purpose was to go to Midgar for Aerith but finds out about Aerith having feelings toward Cloud. It does set how everything around Zack was going to despair from the love of his life having feelings for his friend to world ending events.
The game did make Marlene into someone who knows the future by trying to figure out a way to stop a certain event from happening by having Zack to help in the process as well as making friends with Biggs through her. Zack had to make different choices on who he was going to save in turn made rainbow effects to see a timeline change. Most the choices he made did not end well other than the one timeline where he did not know who to pick. Maybe he went to the main timeline or in the Intregrade DLC timeline. When Cloud did clash with Sephiroth, it showed a rainbow effect that it changed a timeline.
Cloud is still sick mentally through Sephiroth's manipulation. He may be way worse condition compared to the OG Final Fantasy 7 in the next game. Is Tifa going to be enough to put Cloud's mind back together in the Lifestream, or is Sephiroth is going to further break Cloud to change the timeline again? Is Zack going to take part in helping Cloud's mind like Marlene predicted, just in a different way?
Is Zack's sole purpose for being alive is to save Cloud, or is he going to having a bigger purpose in the next game than just saving Cloud? Zack should take a bigger part in the next game instead of what he got in Rebirth.
Marlene may know some parts of the OG future, especially big events like Aerith's death to be fated, but it does not mean she is going to be 100% accurate because she has some knowledge of it.
It is kind of a weird observation I had with Marlene.