In the world of Heartstone, Regency society is stratified less along class lines; Lizzie's unsuitability is rooted in her status as a non-Rider, rather than being a member of the ton. The character of Kitty has been discarded, her throat ripped out by a gryphon before the novel begins. Darcy and Co. are hired to clear out a pack of dangerous gryphons (see: dead Kitty) and Darcy's standoffish nature is more to being a Rider and trauma from battle, rather than disdaining everyone because of their class.
Caroline, here Bingley's twin, is less shallow classist and more hardcore Rider. She's likeable and far less conniving. Lydia is less a flibbertigibbet and is just a young girl who longs for adventure and idealizes the world of the Riders and the Guard. Darcy's and Caroline's interference in Jane-Bingley romance is rooted in Bingley's tendency to fall for women quickly and then have his heart broken, than Jane's class and pushy, impolitic mother. Jane falls ill because she's attacked and wounded, rather than simply having a poor constitution. (Lizzie's trained under her uncle as a healer and Jane survives because of Lizzie's knowledge of plants rather than Jane having to weather a bad cold.) Elizabeth bonds with Darcy's dragon over Lizzie's sketching and her respect for Oldkind. (The dragons can speak.)