I don't think Original Character is a good match for what this is doing either. A lot of worldbuilding-y stories don't center on characters at all, either because they're in fandoms that also aren't character-centric, or because they use the characters to illuminate the world instead of vice versa. The last worldbuilding fic I was given was for a five-minute fandom based on art, where I wanted the fic to be set in the world the art illustrated, but there weren't really characters so much as figures that appeared in the art. That was a fandom where there were characters nominated but they were kind of awkward descriptions of bits of the art and everybody just matched on any/any. And I got exactly what I wanted! The last worldbuilding fic I wrote was for a worldbuilding-heavy epic SF novel that spanned nations and generations and didn't have an intense character focus. That was for a totally blank request, no characters, no optional details. I did canon pastiche themed around one of their generic likes, that had both canon and non-canon characters drifting in and out of the narrative but never its focus, and recip seems to have loved it. Neither of those stories would have sat comfortably with an OC request, because neither of them focused on OCs. (The one I wrote before that I brought in an AU of characters from one of their other requests as a way to explore the canon world, since they'd ok'd crossovers, but based on comments a lot of people thought it was an OC fic.)

I actually read your comment first as a proposal that people who just wanted fic about the fandom, with no character preferences or focus, be allowed to nominate the fandom *itself* as a matchable character, and on thinking about it I really like that as a workaround for AO3's inability to do null character matching well.