There's a greater than 90% chance Distortion World is never brought up or referenced in any way in a DP remake
Nah. I am more confident about Platinum being used far more than Emeralds content back when ORAS was made. Emerald was literally just the Battle Frontier and Rayquaza's involvement, with some minor stuff that was replaced with better alternatives in ORAS. Meanwhile Platinum's changes are far more significant:
Entirely new Area with Story
Battle Frontier
New Dex allowing for a more varied amount of Pokemon
New postgame vignettes, including Charon and Rotom.
All of this can be repurposed for Gen 8.
-Distortion world has a connection to the Energy Source for DMax/Gmax.
-The stat trainers can all get small post game stories for each legendary, including mythicals. This was already somewhat there in DPt, but they can expand Cresselia and Darkrai to include Cheryl for example, or have that little girl be the one with nightmares. They can also act as rivals if they decide to have a tournament in Sinnoh.
-I'm also adamant the Battle Frontier will return, after SwSh has multiple facilties in the London City. That place is gonna be postgame and it shows.
-Rotom story with Charon's role expanded. I'm also fairly certain he will get a cameo in Galar, akin to Colress in Alola.
The lack of a Distortion World wouldn't bug me too much as long as they do a whole post game story for Giratina like Rayquaza got in ORAS.
My biggest worry is that they'll base the PokeDex and gym progression on DP, now THAT would be shitty. I also worry they'll cut the entire post game island because they don't want to do a Battle Frontier. If they don't want to do a Battle Frontier I hope they just put something else in its place there, the whole island doesn't need to die.
Besides the Battle Frontier, that island has the chance to bring the Wild Area to Sinnoh alongside the Underground. In fact Sinnoh has a lot of mini wild area style areas they can make, but the Island is definitely a chance for a Galar Size one.
I don't think they're recycling Megas for Gigantamax forms, because they're two different concepts, and like, brand-wise, we have official artwork and name for Gigantamax forms. Under this premise, Mega Gardevoir and Gigantamax Gardevoir would have the same artwork, etc which just would sound, off. And that also would lead to the false idea that Gigantamax and Mega evolution are the same, so Gigantamax Alcremie is basically its mega evolution too.
Repurposing concepts would be as if Giratina Origin Forme would've suddenly become Giratina Altered Forme's Mega Evolution in Gen VI. That would be just wrong.
So I don't doubt Mewtwo or Charizard are getting Gigantamax forms but I think they will be completely new designs.
They can repurpose Megas as Gigas with the explination that Gigas are the true form of megas. I don't see them bringing back Megas or Z-Moves at all in the future, nor as a postgame feature for competitive, so they have to do SOMETHING with them. For example the moves changing to something else depending on type and if its attack or status is already Z-Moves in all but name, so why not megas becoming Giga Forms? Mega Gardevoir for example becomes G-Gardevoir, Mega Pidgeot G-Pidgeot and so on so forth.