Observation. Just going off of the given number that at least 100 more old Pokémon will come with the crown tundra, that will put Sword and Shield in the realm of about 282 Pokémon still excluded. Further observation, this includes most of the starter Pokémon from other regions as well as the fossil Pokémon from other regions.
Observation, both dlc expansions have a sort of gimmick to the returning Pokémon. The Isle of Armor is simply old favorite returning Pokémon, while the Crown Tundra is putting an emphasis on bringing back all the legendaries.
Observation, 282/2 = 141. A number strangely close to the amount of returning Pokémon with each new wild area.
Observation. Starter Pokémon and Fossil Pokémon would each make great gimmicks similarly to the legendary Pokémon gimmick in the Crown Tundra.
Observation. The Pokémon company loves money.
Conclusion: There will likely be a second expansion pass for Sword and Shield. One new wild area will likely be themed around starter Pokémon, the other around fossil Pokémon.
Further conclusion: Dexit was a waste of time because if the previous conclusion proves true then all of the old Pokémon eventually did come back, and they can all be deposited free of charge via trading from older titles.
Edit: Further observation. Ultra Beasts and Mythical Pokémon would also be great gimmicks. Possible conclusion: Starters and Fossils could be paired together in one pack. The same could be done for Mythical Pokémon and Ultra Beasts.