Do you have more stuff about your Danny Phantom AU?
Admittedly, I don't really plan on doing much with the AU, but plot bunnies do be doing as plot bunnies do, so here's some stuff:
Vlad does have a younger sister, who works with the "Supernatural Shenanigans" Government Department (Brain is thinking that in this universe Guys in White are an outsourced department/where you send the agents who flunked the 'subtly' test and can't quite fire them yet, where basically they have to report to other department which Vlad's sister is part of). She has a daughter that is involved with her own supernatural mischief that would cross paths with Danny and Vlad.
There was a second incident after the college incident which definitely led to the 'absolutely no contact' between the Fenton Parents and Vlad. Does it involve Jack, Maddie, and Vlad going out where they shouldn't have and pissing off a force they shouldn't have which will come back later that Danny will have to deal with? I mean...it would be on brand for the Fentons.
In my AU, I want to play with the idea that Ghosts don't emotionally mature unless another event rattles them in the same vein as their death. That doesn't mean they don't mature in the sense of knowledge or even appearance, it's just more that they're still going to be as emotionally immature/mature as they were when they died even after multiple fights with Danny. So for example, Vlad's ghost form is still going to behave like a college kid from the 1980s while his human self is going to be more mature(ish, this is still Vlad we're talking about)
Sam's family would be more fleshed out, especially with the dynamics between Sam and her dad vs Sam and her mother. Her dad would be an architect that focuses on rehabbing buildings (which would be the catalyst for the first time Plasimus is introduced as Sam would overhear her dad arguing with her mother about one of the buildings/him doing jobs like that, and Sam would jump to the conclusion that he was gonna destroy it, etc) Definitely would need to flesh it out more, but that's kinda where the brain is leading.
I'm sure there's more, but that's what the brain has left me with for now.