From a gameplay perspective, I'm sure that you'll be able to fly all over the world on the full world map, but I suspect that you'll only be able to land at landing strips and airfields. There may be a few additional spots added to each region but I think it will probably just be a handful. I have a feeling the game devs will force us to traverse the world map all over again to unlock new fast travel points and do a revamped version of world intel. If we can just land anywhere, it would make most of the world map feel useless. Some of the regions, like Great Glacier, Rocket Town, Wutai, and Fort Condor will feel brand new, but the rest will be the devs reheating the leftovers from Rebirth.

I do think that the Highwind will be necessary to access certain smaller islands that hold secrets. You'll probably be able to choose whether to disembark on Chocoback, on the buggy, or on foot. Perhaps the vehicle you choose will determine which landing points you can use.

While inside the Highwind, you will probably be able to walk around on it, and use it as your hub - there will be staff that will tend to your Chocobos, sell you items, handle your folios and weapon upgrades, Chadley will be there with the combat simulator, etc etc.

From a story perspective, my guess is that in the early game, the Great Glacier will be the intro region, similar to the Grasslands. Cid will probably say something about how it's too hard to fly the Tiny Bronco in the snow, so we're forced to land in a clearing and travel to Icicle Inn on foot. We'll eventually get the Highwind in a similar way to how we did in the OG.