There will be some ritual for Harry to have Grindelwald's bloodline and for him to become the true heir, because after Sirius is freed he will have some control over Harry as the magically bonded godfather, if Grindelwald is not truly responsible for harry by blood.
I like the concept of him getting voldi's memories however I feel like this will affect harry in the long term cause he is still a kid. dunno if I'm correct tho