HoS interaction with the main game [HoS Spoilers]
While HoS is a standalone expansion it does interact with the main game at several points, I will try to list them all. Feel free to suggest corrections or additional information.
Dead Man's Party
The conversation with Shani would change slightly if Radovid is dead.
Open Sesame!
If you speak to Countess Mignole before the "The Battle of Kaer Morhen" Geralt will say that Vesemir still going strong, later you can talk to him about the countess before the battle, otherwise Geralt will notify her of Vesemir death.
When you speak to Ewald Borsodi at his hideout he will mention Hans of Cidaris, if you took contract on the Griffin in "Creature from Oxenfurt Forest" Geralt will comment that he knew him, later if you choose to free Quinto Hans will be in a sour mood if you discover the original contract and manage to get the full price for slaying the Griffin for him.
Whatsoever a Man Soweth…
If you choose to side with Gaunter O'Dimm and you didn't rescue Ciri from the Isle of Mists you could try to ask him for Ciri location, he will then give you hints on how to achieve the Witcheress ending.
Miscellaneous
You could speak with Ciri, Triss and Yennefer once you arrive to Novigrad about the brand Gaunter O'Dimm gave you.
While HoS is a standalone expansion it does interact with the main game at several points, I will try to list them all. Feel free to suggest corrections or additional information.
Dead Man's Party
The conversation with Shani would change slightly if Radovid is dead.
Open Sesame!
If you speak to Countess Mignole before the "The Battle of Kaer Morhen" Geralt will say that Vesemir still going strong, later you can talk to him about the countess before the battle, otherwise Geralt will notify her of Vesemir death.
When you speak to Ewald Borsodi at his hideout he will mention Hans of Cidaris, if you took contract on the Griffin in "Creature from Oxenfurt Forest" Geralt will comment that he knew him, later if you choose to free Quinto Hans will be in a sour mood if you discover the original contract and manage to get the full price for slaying the Griffin for him.
Whatsoever a Man Soweth…
If you choose to side with Gaunter O'Dimm and you didn't rescue Ciri from the Isle of Mists you could try to ask him for Ciri location, he will then give you hints on how to achieve the Witcheress ending.
Miscellaneous
You could speak with Ciri, Triss and Yennefer once you arrive to Novigrad about the brand Gaunter O'Dimm gave you.