It's only partially related, but one of the more recent episodes of
Smile Precure had a background character that the fanbase took to. When one of the villains is running for student council president (it works in context), there's a single shot of this one girl in the audience.
She was cheering really loudly, and looks somewhat more unique than your average background character. She's already been given a name by fans too. Chinatsu Wakayama, named after a local mascot character who she closely resembles. She didn't affect the plot much (it's clear Reika was going to win reelection, as the challenging candidate teleported off the stage), but she does help to expand the world.
And then there's cases like
K-ON! where every single character in the classroom has a name and small bit of personal information, some of which leaked back into the main series. I love background characters. They can be defined by the fans much more than the main ones, which leads to fan works, which leads to a stronger community.