I think Luna Lovegood could also be included. Her unique perspective and knowledge of the magical world would add an interesting element to the road trip. She might lead them to discover some really strange and wonderful things.
One interesting plot could be the characters stumbling upon a hidden magical village during their road trip. They might discover new spells or magical creatures there.
In 'harry potter is betrayed fanfic', Luna Lovegood might be a character who betrays Harry. Maybe she is under the control of some mind - altering spell or has been deceived by a false prophecy that makes her think she is doing the right thing by betraying him. Also, Hagrid could be a shock betrayer if he is somehow forced to choose between Harry and his beloved magical creatures, and chooses the latter in a way that is seen as a betrayal to Harry.
Obviously, the illithid would be a main character. It could be a mysterious and powerful being that Harry encounters. Also, Hermione and Ron might still be important characters. They could be either suspicious of the illithid or try to help Harry understand it better. Dumbledore might also play a role, perhaps guiding Harry in his interactions with this strange new creature.
Even Ginny could be written as a betrayer in a fanfic. For example, if she becomes possessed by a dark force, she might turn against Harry. Her relationship with Harry is so central in the original story that a betrayal from her would be a huge shock and create a lot of drama in the fanfic.
Harry Potter would probably be one of the main characters. He's the central figure in the original series, so it's likely he'd be involved in this fanfic in some capacity, perhaps as the hitman or someone related to the hitman - style plot.
Obviously, Harry Potter himself would be one of the main characters. He's the hero of the original series and his story would likely continue in this fanfic.
Harry Potter himself could be a main character. He has a history of standing up against injustice, so it's likely he'd be involved in a revolt. Hermione would also be a key figure. Her intelligence and knowledge of the wizarding world's laws would be invaluable in planning and leading a revolt. Ron, with his loyalty and courage, would be right there with them.
Most likely, Harry Potter, Hermione Granger, and Ron Weasley will still be the main characters. After all, they are the central figures in the original series. But there could also be new characters introduced who are either part of their wealthy new circle or those who are affected by their millionaire status.
Well, Ron is also a candidate. He might get tired of all the chaos in the Gryffindor common room or just need some time away from his friends to think. 'Taking the air' would be a good way for him to do that. Luna Lovegood could also be a character who 'takes the air'. She has a unique perspective on things, and being outside in nature might be something she enjoys, perhaps to observe some magical creatures or just to be in her own world.
Most likely the children of the main characters from the original series. Like James, Albus, and Lily Potter. Also, the kids of Ron and Hermione, like Rose and Hugo.