Well, in a 'Harry Potter adopted by Delacours' fanfic, it could be really interesting. Harry would be part of a family that has a different set of values and customs. The Delacours might train him in unique ways of magic that are more in line with Beauxbatons. He could get to know the Veela heritage better, and perhaps it would give him some advantages in his magical abilities. Also, there could be new storylines around his interactions with the Delacour family members and how they deal with the fame and danger associated with Harry.
Harry's character would develop in several ways. He would likely be more cosmopolitan, having exposure to both the British and French magical cultures. His sense of identity might be more complex as he tries to fit into the Delacour family. He could also become more charming and diplomatic, learning from the Delacours' social skills. Additionally, his understanding of love and family could be deeper as he experiences a different kind of family dynamic.
He'd probably be more refined. The Delacours are known for their elegance, so Harry might have better manners and a more sophisticated way of speaking.
In this fanfiction, the Delacours might rescue Harry by using their knowledge of the wizarding world outside of Hogwarts. They could have some secret passages or allies that Harry wasn't aware of. For instance, they could lead Harry through a hidden route in the French wizarding community that the enemies don't know about.
The Grangers. Hermione's parents are Muggles, but if they were made aware of the wizarding world early on, they could adopt Harry. He'd have a stable, loving home environment and would also be exposed to Muggle knowledge, which could be really useful in his magical studies. For example, scientific methods could be applied to understanding some aspects of magic better.
One possible plotline could be Harry learning the unique culture and magic of the Delacour family. For example, he might be exposed to different types of French magic that he wouldn't have encountered otherwise. Another could be his relationships with the Delacour sisters. Maybe he forms a really close bond with Fleur, like a brother - sister relationship, which changes the way he views relationships in general. Also, his upbringing by the Delacours might give him a different perspective on the wizarding world politics, as the Delacours could have their own views and influence on him.
Well, it could involve Harry growing up with the Delacour family's unique culture. Maybe he'd learn different kinds of magic that the Delacours are known for. He might also have a different personality, influenced by their flamboyant and refined ways.
Obviously, Harry Potter himself is a main character as he is the one being rescued. Then there would be the Delacours, mainly Fleur Delacour. She is a strong and resourceful witch. Her family members, like her sister Gabrielle or her parents, could also be involved in the rescue. They might work together as a family to save Harry.
In this fanfic, Harry's character development could be quite significant. Being raised by the Delacours, he would be exposed to a different set of values. He might grow to be more confident in his magical abilities as the Delacours could encourage him more freely. Also, his sense of family and loyalty could expand as he becomes part of the Delacour family. He may learn to be more diplomatic, dealing with the different personalities within the family and their friends. This new environment would shape him into a different kind of wizard compared to the one we know from the original series.