Question: My story is very much dependent on its being told in first-person (as I tend to write character better than plot). However, given that I'm aiming for a "high-concept" story, there'd potentially be a lot of the story world left unexplored. Is there a way to broaden the scope of my overall story while still keeping a sharp focus on my first-person POV?
By definition, a first person narrator can only share with the reader things they know or perceive. So a common approach is to have a main character who starts out knowing only a small part of the story world. The reader then learns about the broad world as the main character discovers it.
If there are certain aspects to the overall story that the main character will not discover but you want the reader to know about, your only option is to switch to a different point of view.
Note that you don't necessarily need another first person narrator (though that is a possibility).
You could introduce a character narrator, perhaps someone from within the story world but at a later date -- a historian for example who comments on the main character's personal account, or perhaps a computer AI that gives a report to the reader.
You could also have a pure omniscient POV who appears in separate short chapters to fill in the reader. (But this risks creating infodumps, which are not very popular today.)
You can also bring in other perspectives through documentation. For example, in The Hitchhiker's Guide to the Galaxy
Douglas Adams inserts excepts from the guidebook at various points to give the reader a broader perspective on the story world. You could similarly use newspaper or encyclopedia excerpts (or perhaps transcripts of video broadcasts) for a similar effect.
Another example from genre fiction would be the Watchmen
graphic novels, which include a wide variety of documents (articles, letters, reports, etc.) written by different people to fill in the backstories or give perspective.
Of course, such breaks or diversions from the main character's POV will weaken the connection between the main character and the reader, since they will give the reader a broader perspective than the character. You have to decide how to balance the inevitable trade-off.