The game engine for Pokémon Sword & Shield will almost certainly be used for an upcoming remake of Pokémon Diamond & Pearl, which raises the question of how the new mechanics introduced in the Galar region would work in the Sinnoh region?
Pokémon Sword & Shield introduced the Wild Area to the series, which is a massive open piece of land that is teeming with wild Pokémon that appear on the overworld map. The scientists in the Galar region were also the ones who discovered the Dynamax and Gigantamax phenomenons, which allow Pokémon to briefly transform into giants.
A lot of work went into designing both the Wild Area and the Dynamax mechanics for Pokémon Sword & Shield and it's unlikely that Game Freak would ignore them if remakes of Pokémon Diamond & Pearl were the next entries in the series. The Sinnoh region and its Pokémon Trainers would need to be changed in order to use these new features, so we're here to speculate on how these could appear in the games.
The Wild Area(s)
The Wild Area is one of the most popular new additions to the Pokémon series, as it gives a glimpse of the Pokémon MMORPG that fans have waited to see for years. The Galar region was designed with the Wild Area in mind, as it takes up a good portion of the center of the region.
The problem with bringing a Wild Area to Sinnoh is its existing map, as the center of the region is dominated by Mt. Coronet, which is a location that is central to the story and needs to be visited at specific times.
The most obvious solution is to create several small Wild Areas throughout the region and have level-appropriate Pokémon in each. The Great Marsh near Pastoria City already acts as the Safari Zone for Sinnoh and it could easily be repurposed as a Wild Area. The snowy region below Snowpoint City would also be suitable for a Wild Area, as would the unused wilderness west of Canalave City.
The Gym Leaders
The Sinnoh Gym Leaders were already reworked in the transition from Pokémon Diamond & Pearl to Pokémon Platinum, but they would need to be reworked again for the remakes in order to account for Dynamax and Gigantamax Pokémon, as well as adding new additions from the Galar region.
Roark is a mining-themed Gym Leader who specializes in Rock-type Pokémon, so adding a Rolycoly to his team would seem like a natural fit. He could also use a Gigantamax Coalossal during his rematch.
Gardenia uses a team composed of Grass and Grass-type Pokémon, so the Grookey and Gossifleur lines could easily be added to her team.
Lucario is one of the most popular Pokémon in the series, so it's surprising that it didn't receive a unique Gigantamax form in Pokémon Sword & Shield. Maylene's Gym battle would be the perfect place to introduce a Gigantamax Lucario, as it's the strongest member of her team.
Gyarados is another popular Pokémon that was skipped over for a Gigantamax form. Crasher Wake could switch out Floatzel for a Gyarados in his team and use a Gigantamax version of that for the battle.
Fantina underwent a lot of changes in the transition from Pokémon Diamond & Pearl and Pokémon Platinum, which resulted in her Gengar being downgraded into a Haunter. Gengar already has a Gigantamax form, so that could easily be brought back and switched with her Mismagius.
Byron is another mining-themed Gym Leader, but he prefers to use Steel-type Pokémon in battle, so the Rolycoly line wouldn't work with him. He could easily add a Duraludon or a Copperajah to his team, as they both already have Gigantamax forms.
Candice could easily swap out her Mr. Mime for a Galarian Mr. Mime or Mr. Rime. Her signature Pokémon is either Abomasnow or Froslass depending on the game and either of these would be a good candidate for a new Gigantamax form.
Volkner only uses pure Electric-type Pokémon in his first battle, so a Boltund or a Pincurchin could easily fit onto his team. His signature Pokémon is either Luxray or Electivire depending on the game and either of these would be a good candidate for a new Gigantamax form.
The Elite Four
Aaron of the Sinnoh Elite Four really needs to drop Drapion from his team, as it goes against his Bug-type theme. Drapion can easily be replaced with an Orbeetle (which already has a Gigantamax form) and make either Heracross or Vespiquen his signature Pokémon and give that a Gigantamax form.
Bertha's team doesn't need too much tinkering, but Hippowdon or Rhyperior should get a Gigantamax form in order to match the other members of the Elite Four.
Flint's team is also solid, or at least the Pokémon Platinum version of the team is. If Volkner's Electivire can Gigantamax, then Flint's Magmortar should also do so, referencing the rivalry between these two Pokémon lines.
Lucien's Mr. Mime could easily be swapped out for a Mr. Rime and either his Alakazam or Gallade should be given Gigantamax forms.
The Garchomp line is missing from Pokémon Sword & Shield so it would likely return in a big way for a Pokémon Diamond & Pearl remake, as a Garchomp is Cynthia's signature Pokémon. It's also a strong candidate for a Gigantamax form, seeing as it's likely going to be the final Pokémon that most players are aiming to defeat. Cynthia and her team are one of the most fearsome challenges in the Pokémon series and it wouldn't need any tweaking for her to be as powerful in the remakes as she was in Pokémon Diamond & Pearl.