First person has a flexibility that a lot of writers don’t consider. Sure, you have some parameters. Your narrator can’t, for example, know with certainty what another character thinks and he can’t witness what happens at a dinner party he doesn’t attend. Still, you can reach outside one character’s experience when writing in first person. You just need to get creative.
One option is first person serial point of view, where you give a series of first person perspectives throughout the novel. You might section this off by chapter, switching to a new first person narrator at each chapter break. The first and final chapters of Russell Banks’ The Sweet Hereafter are from the perspective of Dolores Driscoll, the woman driving the school bus when it swerved off the road and plunged into the icy water, killing many of the town’s children. Other characters narrate the intervening chapters: Billy Ansel, who lost his children in the accident, Mitchell Stephens, a negligence lawyer who comes in from out of town to stir up a law suit, and Nichole Burnell, a young girl who survived the accident. This approach works well to tell the town’s story in the aftermath of this loss.
Another option is to let your first person narrator speculate or imagine in a way that brings other characters’ experiences to life. Jeffrey Eugenides’ Middlesex is narrated by Cal, an intersex character who tells his own story as well as a larger story that reaches across generations. Cal is the sole narrator, but quite a bit of the story happens before his birth. While those sections are in third person, the understanding is that Cal has imagined his way into these moments. This can be a bit of a leap for the reader, but if you craft the voice and environment so that the writer can willingly suspend disbelief, you can show moments outside the narrator’s personal experience and stay true to the point of view strategy.
This same approach can also be used in a more limited way. In F. Scott Fitzgerald’s The Great Gatsby, for example, the first person narrator, Nick Carraway, imagines his way into the moment when Gatsby and Daisy shared their first kiss. He wasn’t there, but he’s clearly thought about it and he makes it a vivid scene for the reader.