Theme. Main Idea. Topic.
While you could argue that all writing contains a theme, I’d argue that if you were drawn to this page maybe what you are wanting to create is focused on ideas.
I'm not specifically asking you to write a classic academic essay here (although it could be a fun exercise to create an academic style essay in the voice of a character you've been imagining or on a topic that fits in with a poem you’ve been drafting etc.). Non-fiction can take many forms: biographies, memoirs, journalism, and technical writing are a few sub-genres that fall within this category.
If you aren't sure whether or not you really have a solid theme, why not play with the word a bit more as well? Think about all the definitions that fall under the word theme (from the Oxford Online Dictionary):
the subject of a talk, a piece of writing, a person's thoughts, or an exhibition; a topic
an idea that recurs in or pervades a work of art or literature
a prominent or frequently recurring melody or group of notes in a composition
a piece of music that frequently recurs in or accompanying the beginning and end of a film, play, or musical
give a particular setting or ambience to (a venue or activity)
any of the twenty-nine provinces in the Byzantine empire
an essay written by a student on an assigned subject
and some other older uses of the word theme in relation to the study of linguistics that I want to revisit because I feel like I ALMOST understand these concepts! You can go down that rabbit hole yourself if you'd like.
What to do next?
How about trying a tried and true pre-writing technique?
7 - Free-writing
9 - Outlining
11 - Questioning