This is not firsthand information, some details may have been lost in translation.

* Titles are Decade Diamond and Galactic Pearl

* New difficulty option in the settings. This does not change NPC Pokemon or their levels, rather it simply turns off the exp share.

* The games are fairly basic in terms of scale. There is no wild area or controllable camera. The games pretty much look like Lets Go Pikachu/Eevee.

* The Poketch returns as Rotom Watch. Apps are unlocked as you progress, showing different kinds of information in the corner of your screen. There is a vs seeker app.

* Following Pokemon return and take the place of HMs once you have the corresponding gym badge. For example, Rhyhorn, Machop, Geodude can smash rocks after beating the Oreburgh gym. Starley, Kricketot, Zubat can cut trees after beating the Eterna Gym.

* Contests return and are very similar to the system in Omega Ruby/Alpha Sapphire. Cosplay Pikachu returns, and Pachirisu gets cosplay outfits as well.

* Making Poffins is a motion controlled minigame, similar to cooking Currey in Sword/Shield. There is a PoffinDex.

* The underground returns as an online hub with plenty of minigames. Secret bases, mining, and mega raids can be done here.

* Mega evolution returns. Apparently megas will continue as a staple of the remakes. New megas include: Infernape, Empoleon, Torterra, Honchkrow, Togekiss. There may be more, these are all I know of.

* Dialga and Palkia have new forms similar to Primal Reversion in Omega Ruby/Alpha Sapphire.

* There is no Distortion World and Giratina can be found in Turnback Cave.

* Arceus doesn't make an appearance.

* The national dex doesn't return, the dex is only slightly expanded from Platinum.

* No battle frontier, but the battle tower returns.