... I do think Maggie will have some sort of ability/talent at some point; Mom was a half-Ramp, Dad's a wizard, so the odds are stacked in favor of her being magically-inclined ...
Rampires are
made, not born. It's almost like a psychic version of the parasitoid wasps -- an adult Rampire can implant an immature Ramp-spirit, when it attacks a suitable mortal host. They cannot reproduce from the hosts' mating (Whampires are the opposite, reproducing with host-matings but unable to spread by attacking/infecting others). I don't think we have any (canonical) reason to think Susan would have "passed along" any special powers/abilities. Is there WoJ on the matter???
As for Harry's bloodline... I think it almost as unlikely. Harry says inheritance of magical powers is poorly understood, but mostly (when it happens) follows "Salic Law," i.e. the mother's bloodline. Harry's mom was a strong magician, but his dad was a muggle. That said... WoJ was that Eb's wife (Margaret LeFay's mother) was herself a muggle; so if Harry's birth is a case in point, the prior generation proved the opposite point.
Finally, of course, is the question of plotline & story. Maggie having strong talent, and needing training, will add to the complexity of Harry's life, so
of course Jim will write it that way! Unless he decides not to (in order to confound the fans and mess with expectations, I'd presume).