Final Fantasy 7 Rebirth's open world is apparently inspired by Final Fantasy 15, Horizon, and The Witcher 3.
At Paris Games Week 2023 earlier this month, Final Fantasy 7 Rebirth co-director Naoki Hamaguchi fielded questions from French outlet FF Dream about the upcoming game. When asked about Rebirth's open world, Hamaguchi began by saying Final Fantasy 15 gave the development team experience needed to work on Rebirth's open world.
Additionally, Hamaguchi also pointed to The Witcher 3 and the Horizon series for touchstones for Rebirth's open world. The co-director doesn't specifically point to Horizon Forbidden West as an inspiration for Final Fantasy 7 Rebirth, so don't automatically assume Square Enix is looking to the more recent of the two games for inspiration.
From the sounds of things, then, Final Fantasy 7 Rebirth is going to be a pretty huge game. Square Enix already revealed last month in October that it'll take around 40 hours to beat Rebirth, which already positions the new game as bigger than 2020's Remake, if you're going purely by the length of the main story and critical path.