I'm a Freelancer that have worked recently with GF and is still close to several employees. The plans for the 25th anniversary are somewhat similar and somewhat different to what you could expect.

>D/P Remakes called Project Alpha, plot will be changed and will now revolve around Arceus.
>Underground returns and will inherit free camera and multiplayer
>lets you collect Kanto/Hoenn/Sinnoh/Unova fossils
>No wild area on the surface
>HMs are replaced with something pretty similar to Alola mechanic
>Celestic Town has now an Arceus Temple like the one in Jewel of Life movie
>Giratina won't appear till post game, like original D/P
>Megaevolutions returns but the base stats boost is 50 instead of 100
>Dialga, Palkia and Giratina orbs will grant them a Primal Form

As of Gen 9, Game Freak is planning on a huge leap forward and a lot of external manpower is being added. Expect gen 9 to be open world to the core.