Ok, assuming the rumor of GameFreak trying to shake things up is true then I feel like they would take some inspirations from the games that shake things up, Mario Odyssey and BoTW.

I feel like we are going to get giant Pokémons or properly proportion Pokémon like Groudon in the anime. I say that because of the Divine beast in Zelda and the huge Titans in Xenoblade 2.

I still think they will have a free roaming area. But like other open world, you're allow to go to area you haven't been even if the Pokémons in that level have a higher level than yours.
Truth be told, for me, it can be linear but it have to be seamless. You know how with twilight princess you have a transition loading period but in breathe of the wild there isn't any, unless you do insta-travel.

One last thing, they will bring back free fly mode. Where you're flying on a Pokémon's back and you're in control. This time it won't the area you're in isn't a fake area like the last game that had it or like skywards sword. It'll be the same world, again like Breathe of the wild when. They do the magnesis glitch.

And maybe the game will be more streamline... I don't know how they will pull it off but Mario did it.