In the DLCs you wil be able to encounter over 230 familiar Pokémon that had not appeared in Pokémon Scarlet and Violet.
Here you can see all the returning and Home Transfer Pokémon (based on datamined information)
If you purchase now The Hidden Treasure of Area Zero, you'll receive the New Uniform Set that you can immediatly use in Pokémon Scarlet and Violet.
Plus, you'll get an Hisuian Zoroark that cannot be obtained through normal gameplay. It knows the move Happy Hour, its Tera Type is Dark and It has the Charismatic Mark.You have to purchase the expansions by October 31 to receive the bonus code.