Region:
>Platinum regional dex
Improved Sinnoh's small and lacking dex
>Improved towns
Adding to and reimagining some of the towns in Sinnoh would do a lot for the unremarkable town design
>Gym order
What was done in Pt to improve the pacing slightly
>Pokemon improvements
Some Pokemon that previously lost megas receive them as new evolutions after the Nat Dex is received. Rowan and Sycamore give a quest and require a new item to evolve permanently. Other Pokemon get stat buffs/moves, like Carnivine.
>Marsh
Great marsh unlocks wild areas for regional forms from Alola and Galar as well as other Pokemon from elsewhere

Story:
>Platinum's changes
All of Platinum's changes should be retained for Galactic and pacing
>Expanded Galactic
Team Galactic is rather lacking even in Pt and could be expanded upon similarly to Magma/Aqua in ORAS
>Pacing changes
Altering the pacing so there aren't so many gaps between the main story and the Sinnoh league would help keep the game feeling less like a blur towards the end
>Giratina
Including Giratina in the Distortion World as it's one of the more unique locations Pokemon has.
>Gym Leaders
Similarly to TG, the gym leaders in Sinnoh could use some extra fleshing out to make them feel more alive and relevant beyond a battle. Perhaps having them give side quests or backstory or playing any role would be beneficial since besides Fantina and Volkner, they're mostly forgettable
>Stark Mountain
Retain Pt's post game Heatran story and expand upon it
>Stat trainers
Give them some extra story and flesh them out beyond just being a partner for a little bit
>Lake trio
Improve them so they are memorable and not just generic legendaries with good music, perhaps expand on their lore and give them some unique flair like moves or abilities or forms
>Barry and Dawn/Lucas
Barry should receive some extra characterization, while the counterpart could get a treatment similar to Wally where they have a culminating battle and some more story relevance.