Honestly no idea. The only Featherington family member mentioned in the book that wasn't introduced in the show is the youngest sister Felicity. But Mr. Featherington is already dead by the start of the series and not mentioned all that much. I can't even think of any of the books' chatacters they could try to introduce in this capacity. Going by future heroes, Phillip has already been brought in, which leaves John and Michael, Francesca's future husbands, and Gareth, Hyacinth's future guy, who's also Lady Danburry's nephew. I don't know how any of that would work. Even as for villains, of the male variety, I think there's an old guy in Gregory's book, but can'št really remember anything about him. That's really all I can come up with. Whoever it turns out to be, they'll have to change a lot about the character, but they've already done so with Berbrooke, so I can't see that stopping them.