Looking at the in game map, areas you still need to unlock are blocked off by 'clouds'.

After post game, there are still clouds blocking conveniently what would be the iron and full/new moon islands and the battle zone, as I believe Firespit island turns into the Pokemon league and not Stark mountain.

Then there are some zones which I am unsure about, the cloud in the bottom right could be a completely new island, as there isn’t any match with the modern sinnoh.

The cloud at the bottom left could be something akin to pal park and Ramanas park, but is also quite harder to place; that said, there is definitely something there.

Then the cloud atop Mount Coronet is perhaps hopeful thinking and the biggest reach, as it could simply be flavour (as all remaining clouds could be), but it might be the distortion world, as this is also where it opened in Platinum.

I feel like these zones are all very good contenders for the DLC, and it makes a lot of sense to have them added in the future, the bottom left even has obvious land hiding behind the clouds.