Final Fantasy 7 Remake is taking the story of the original game in a new direction, but it's possible characters from FF7 spin-offs could appear in later entries. There is one character who may not be able to be included in FF7 Remake, however, due to issues regarding his design.
Genesis Rhapsodos appeared in the secret ending of Dirge of Cerberus: Final Fantasy 7 and had a major role in Crisis Core: Final Fantasy 7. He was a member of SOLDIER and was experimented on as part of the Jenova Project. Genesis was one of the strongest members of SOLDIER, yet his body started to degrade. He defected from the Shinra Corporation and began kidnapping scientists so they could prevent his body from falling apart. In the games, Genesis' body is healed by the goddess Minerva, and he is taken by members of Deepground. He appears during the secret ending of Dirge of Cerberus, where he takes the body of Weiss the Immaculate.
Genesis should still be alive during the events of FF7 Remake, and he might have been a strong candidate for a cameo or a role in the story, but celebrity likeness rights could cause problems with his inclusion. Deepground and Weiss will appear in FF7 Remake Intergrade, but it's unclear if Genesis is even allowed to be there with them.
FF7's Genesis Is Based On Japanese Musician Gackt
Genesis' appearance is based on Japanese musician Gackt, who also voiced Genesis in both of his games. Gackt was on board for Dirge of Cerberus and Crisis Core, but Square Enix would likely need his permission to reuse the design for the FF7 Remake series. Genesis has appeared in Final Fantasy mobile games over the years, but these have been chibi/cartoonish versions of the design that don't mirror Gackt's likeness as closely.
The very nature of the FF7 Remake series allows developers to completely rework Genesis' design so that he no longer resembles Gackt, as the changing timelines at the end of FF7 Remake could be used to explain the difference. Characters also have new voice actors in FF7 Remake - another reason to not need Gackt. The producers of FF7 Remake could easily bring Genesis back into the story for future entries in the series, but they would have to change his design if Gackt refused to allow his likeness for use. Luckily, there are plenty of ways developers can go about changing Genesis' appearance while keeping Crisis Core backstory intact.
Final Fantasy 7 Remake: Intergrade is coming to PlayStation 5 on June 10, 2021.