Alice Notley notes that when she was writing poems about being pregnant and then a young mother and taking care of kids, etc... that that kind of poetry, based on that experience, had never been written before. I'm not going to dispute that because what matters is her perception of this. She is probably right, and any counter example would be the exception that proves the rule.
I mentioned that Cummings poem about Buffalo Bill is the first poem ever about the death of a pop culture celebrity. I can't prove that either, and if someone has an earlier example I would stand corrected.
Another first might be Keats' "This living hand, now capable of earnest grasping...." This is the first poem I know that has that quality of emotional rawness to it. It's not even in other Keats poems.
Of course, once you see that, you can go back and find precursors to any thing that seems to be a "first," like Borges's precursors of Kafka. Kafka is a first: there is not that particular feeling in literature before that.
That would seem to be the important thing, because all the archetypes have always existed, the narrative structures of quest or homecoming, the particular rhythms of plots with up and down movements, problems being posed and resolved. You can't be original with that, but new kinds of feeling can come into being. You can have the -esque of Kafkaesque. (And I don't mean the Kafka cliché, but the feeling one gets from it.).
What are some other firsts? I associate this with writers who "cut through" and identify new forms of feeling, make them recognizable.