Pokémon Diamond & Pearl remake idea:

In Pokémon X & Y mega evolutions were introduced, and more mega evolutions were added in Omega Ruby and Alpha Sapphire.

So since Alolan forms were added to Pokémon Sun & Moon because pokémon adapted to the different environment, what if Dialga, the time warping pokémon, went back in time and you could catch fossil pokémon as their original forms and typing before they became rock types;
Palkia, the pokémon that controls space, could teleport you to a different planet where pokémon from space adapted differently to that other planet.

And Giratina, the pokémon banished to the distortion/reverse world, could have pokémon or ultra beasts that have adapted to the distortion/reverse world that were also banished to the distortion/reverse world, or the distortion world could have entirely new pokémon that live only there.

This would also be like the entirely different dimensions the ultra beasts live in Pokémon Ultra Sun & Ultra Moon, all with either a ghost or dark type added to them.
So this would tie in very well with what has been going on in generation 7 of core RPG Pokémon games thus far.