Plenty of info for what GF has planned for the future of pokemon

Let's start on the DLC
-Megas are indeed returning but there'll be no new mega evolutions but will be obtainable through special raids where pokemon are both dynamaxed and mega evolved which will allow you to obtain the mega stones
-Some new features coming are brand new apricorn balls that are able to made with new apricorn varieties.
-The isle of armor will make sure that every single previous starter is obtainable within it
-The crown tundra will make sure that every single previous legendary is obtainable within it
-New item called G-max wishing piece which when used on a pokemon who has a G-max form it'll give the pokemon the G-max factor and when used in a den it will summon a guaranteed G-max raid(they're very rare items that can only be obtained in raids or by completing the towers of two fists after the Isle of Armor story)
-Players who bought both dlcs will be rewarded with access to special areas where certain pokemon are available as well as being able to get the other version exclusive legendary(zacian for shield and zamazenta for sword)
-There will be a sword and shield bundle with the expansion pass that will release with some extras(nothing too big)

Regarding Diamond and Pearl remakes
-I can say for a fact that, yes, they are indeed coming soon
-There is no dlc planned for the games
-A new type of raid battle will replace max raids in this game
-No national dex but GF will make every pokemon available in SWSH tradable to the remakes and SWSH will have an update where the pokemon in the DP dex will be tradable and usable in SWSH