I do want an open world, and I hope they do an improvement and use today's open world elements. Add things like bounties, tons of new side quests and hidden monsters, gear and weapons. Make it modern while keeping the core ff7 intact
I just think that a true open world in the sense of XV would be a complete distraction... not only for development time, but for the focus of the gameplay itself. The original game is already a massive undertaking to translate into a modern game, and most of it occurs in towns/dungeons. Turning the original's tiny map (in gameplay terms) into a huge sprawling endless environment will really cause the game's focus to go into the weeds.
Let's think of the area around Midgar. I could appreciate turning the area with Midgar, Kalm, the Chocobo Farm, and Mithril Mine into a nice big environment on the scale of FFXI, FFXII or FFXIV's regions. Even Lightning Returns' wildlands might be a good comparison here. So if that means "open world" to you, then I'd agree with it.
But something like XV or GTA's open world would ask that you actually make a continent to scale, with huge distances to cross, and that seems like a distraction to me. Remember that FFXV takes place on a single continent because of this kind of scale. That kind of open world just seems like a diversion from the core scenario to me, when all the meat of the game happens in Midgar, Junon, Gold Saucer, Mt. Nibel, etc, and not on the old world map.
Maybe they could do it, but it would only be by the grace of being cut into multiple episodes. That kind of open map would definitely not be in Episode I.