I worked for a publishing company and we made videogame guides. At a point before release a handful of people are usually picked to actually play the games but also bound to a firm contract while the rest of the workers are tasked with formatting the pages. Our company went bottoms up due to covid restrictions so Im no longer bound to NDA. Yay me!

>Pokemon Eternal Diamond Infinite Pearl
>Game has Dynamax and Raid Dens, no Megas/Z-Moves
>Underground had been changed. It is basically the wilds but underground. A giant underground cave system with wild Pokemon and nodes to mine items from similar to Monster Hunter. Pickaxe minigame removed. Poffins removed. No multiplayer underground functions. Camp is a part of the underground.
>Special Gigantamax forms for Palkia Dialga and Giratina
>Postgame Islands have been almost entirely removed. There is only a ship to go to the Battle Tower. Pokemon like Heatran, Darkrai and Shaymin will be a part of episodic DLC packages that will be $9.99 a piece
>Postgame is an Omega episode where a new NPC trainer and you solve a mystery about dynamax energy creating reality warping disasters and you eventually find out its Eternatus. Then his whole backstory is explained where he was originally part of the Creation Trio but felt wronged by Arceus because he didnt have a gift to rival Dialga and Palkias Time and Space mastery so he turned on Arceus. Arceus was angered by this and the two had fought for a time before Arceus obliterated Eternatus with a meteorite that crash landed on Earth
>Arceus Eternamax form is basically Arceus' true form with 255 stats across the board (Also not playable)