Prosaic could have been a good call but the writer stresses that there are only a few lines scribbled on a napkin so underdeveloped is a better fit.

prosaic means most similarly "boring or cliche." If his plot was boring and cliche one would not expect a hollywood producer to pick it up. On top of that, the hyphenated portion is expounding on the word in the blank. Since it is saying "lacking detail" rather than "the details were boring," we are looking for a blank that means something like undeveloped rather than prosaic.

"more than a few basic plot points scribbled on a napkin" well it can also represent something which is dull or prosaic. why it could not be an option?

Although his latest project was relatively _______ —little more than a few basic plot points scribbled on a napkin—the veteran screenwriter easily sold the story to a major Hollywood studio.


The sentence starts with although which means a contrast is ahead of you. Moreover, the part of the sentence is there between the dashes to put emphasis to what the blank is

So, his job was written or a napkin nonetheless was probably outstanding or considering the fact that even though the writing was more than a scratch on a paper probably it was fairly complicated and not so simple
, something like that because then he sold it to a major studio


And because the words between the two dashes says" more than a few basic plot " the paper was relatively underdeveloped

I hope this helps