I don't know if it's going to be on rails, but that might not be a bad thing considering the scale of the game. This is something new for the final fantasy series, I don't believe that they have a story quite like this one (based on reality with fantasy). I mean if you think about it, Kingdom Hearts, one of the most successful square enix game is kind of on rails. You go to a world, complete task, repeat. It might not be a bad thing, I guess there is only one way to find out.
There were lots of games like that before rpgs, in fact they were the vast majority before ultima video gamed table top rpg's, and Nintendo refined persistant world action adventure games and made history.
RPG's full of large worlds without rails and leashes, games that played mechanically like action games, but did more than simply traveled left to right killing everything became popular because people wanted more from their experience than mindless passive entertainment.
Back when Squeenix was called squaresoft, they were part of the pioneers creating the path of more than just passive entertainment. Now they clearly arent. They believe there is far more money to be made off the low hanging fruit.
For the record, i have a rather poor opinion on kh. Its a poor action game, consisting of simple, sloppy, repetitive action, its a poor rpg, simplified to the point of insult in all aspects, from stat progression to world/progression design, and the story is absolutely cringworthy.
And this looks worse on all fronts save for visual presentation and story.
I dont want to go on a theme park ride that dramatically depicts the things I used to actually do. To me, that is the worst thing you can do. There is literally no reason to spend money on a game like that, simply watch it on youtube.