GPT is going to be rather poor at priming people for an amazing ending by seeding the ideas and building them into the narrative. Though if you're directing it with enough granularity, you could tell it to do that just like you'd tell yourself to do that when you're doing the writing yourself.
But then you're becoming the executive writer. On a granular enough level, the most ultimate executive control of GPT would be picking individual words, just like you were writing them yourself. Once you want to step away and tell it to do the writing for you, you drift more into the GPT-nature to the point that it becomes obvious.
"I want <my favorite novel> rewritten in the style of <favorite author> but please focus more on <interesting theme>." I see so many possibilities. Passionate readers could become more like curators, sharing interesting prompts and creations.
Because someone mentioned Kafka: I'd like to know what Kafka's The Trial written in the style of a PKD novel would be like.
AI that's trained on their style could give me what I want.
GRRM fans also should probably think of the ways to feed ASOIF to the AI if they want to know how it ends.
LLMs will reach the same level sooner or later.
But your point is of course valid, it would not be a 'work of Shakespeare'.
That is possible yes, but only within a limited interpretation of 'the style of Shakespeare'. It could only draw from the lexicon used in the existing body of Shakespeare works, and perhaps some other contemporary Elizabethan playwrights. It wouldn't include any neologisms, as Shakespeare himself invariably included in each new play. It couldn't be a further development of his style, as Shakespeare himself developed his style in each new play. So it would be a shallow mimicry and not something that Shakespeare would have produced himself if he had written a new play (based on a 21st century authors plot).
I personally wouldn't find that interesting. I acknowledge that you wrote only 'mildly interesting' and yes, it could be mildly interesting in the way of what an LLM can produce. But not interesting in the sense of literature, to my mind. Frankly, I'd prefer just to read the original new play written by the living human, if it was good. (I also prefer to not ride on touristic paddle-wheel boats powered by a diesel engine but with fake smokestacks.)
Sorry for the strong language but this is a ridiculous line to take. A 'work of Shakespeare' is not even remotely open to interpretation as being something produced in the 21st century.
I expect you could summarize the preceding, already generated story within that context, and then just prompt for the next chapter, until you reach a desired length. Just speculating here.
The one thing I truly cannot wait for is LLM's reaching the ability to generate (prose) books.