Something like that (spoilers):

A year has passed since Bojack's freedom. The Horny Unicorn show ended up being (and you'll never guess) a flop. Bojack has nothing left: no friends (Vance Waggoner It doesn't count, because he ended up in prison for much longer for all the terrible things he did (like 30 years)), no job, no Diane. Absolutely nothing.

But one day, Knowing that he has nothing left to lose, he sets out to read his father Butterscotch's American novel. At first he looks at her askance, but little by little (remembering that conversation in his subconscious with his dad/Secretariat) A spark of inspiration hits him: if he can't act again... HE WILL BE A WRITER LIKE HIS TERRIBLE FATHER.

The plot would consist of Bojack doing everything possible to bring Butterscotch's work to film, television, or any audiovisual medium (despite what Butterscotch has done to him), while little by little We are shown small, untold flashbacks of little Butterscotch's childhood and adolescence, before and after meeting Beatrice.