In a recent interview with Kitase he talked about some new details in the remake. First off, he said the length of the game episodes are looking to be on par in length with the size of a Final Fantasy XIII game. As for the inclusions, if there are areas he can use, he is looking at either having mentions of some of the spin-off characters and places, or having the people or places make an appearance, at least, for the characters in the spin-offs that were most popular.

Who would you want to see appear in the remake, if any spin-off character? I would like to see more of the zack fair flashback, or in fact have it be like a playable part of the game as like an intro to the game and then a flash forward to the intro of FFVII. He did have a metaphysical appearance in Advent Children like Aeris did so I would also like to see him appear in some instances like that watching Cloud and co's progress and commenting on what he witnesses.

I also would like maybe a brief mention of Lazard after the events of CC, if any of you played the Crisis Core prequel you know what I mean. If I were making this remake, I would make some connection so the continuity makes sense in the FF7 compilation.

If anyone else, I would also like to know what's up with Cissnei. That's about all I can remember from the spin-off prequel wise.

More in the article if you want to see the Kitase interview.
Final Fantasy VII Remake May See Some Areas And Characters From Its Spinoffs - Siliconera