They are going to sell FF VII in multiple games, similar to FF XIII. They must have modelled new areas to explore, areas that were previously unseen in pre rendered backgrounds. They can even have added new side quests or modified the plot in some ways to acommodate a larger world. In the original game you visit one reactor in Midgar and just one section of the whole thing, what if the whole structure is now accessible to the player?
Realtime combat imposes a new challenge. What the other chars in your party do while you are controlling one char? The AI controls them. In turn based system there is no AI to control them. Another thing that they must have changed is the random encounter system. It's really annoying to be walking and suddenly a battle takes place. That screen transition between the world and the battle is annoying. There was even a yellow materia that made the encounters less frequent.
They haven't showed anything about the world map. In the old days an open world with the scale of GTA or The Witcher was impossible, but now they can and have done it with FF XV. This forces some more gameplay changes as you can't just walk out of Midgar and run to Kalm in one minute. That or they abolished the world map.
Aerith's death can now have blood if they aren't censoring anything.
If they are succesful with that remake, they are probably going to make more. FF8 anyone?
There are some reports that Mark Cerny invested in ultra fast SSD for the PS5, claiming it to be faster than what is available for PCs. Sounds like they included a very large DDR cache, much larger than what is available in current SSDs.