After investigating Harry’s new Firebolt, Hermione becomes a Bristlehead, like her Gearhead father before her, and is now much more invested in Quidditch.
After checking Harry’s mysteriously gifted new top-of-the-line Firebolt, Hermione becomes very interested in the production and advancement of brooms. Suddenly she’s asking to look at the school brooms, as well as the Griffindor teams brooms as well. And after a while she even improves some of them, eking out more speed or stability where required. She even starts making her own models, essentially becoming the equivalent of a fanatic F1 mechanic who drives like a grandma at all times, Having Harry do any testing that can’t be don’t above a crash pad in her workspace (first an abandoned classroom, then an expanded space in the Room of Requirement.) which leads to a sweet little friends-to-lovers romance that involves a lot on collisions between the two from faulty brooms, leading at least one person to wonder if she or Harry are a harem protagonist.