Man, you gotta look at the sheer volume of tooth-rotting fluff out there first—it's a whole genre. I think those stories exist because the original comics show this unshakable, creative partnership, but George and Harold are, like, what, nine? So a lot of writers take that foundational loyalty and age them up, putting them in high school or adulthood to see if that bond changes or deepens. It becomes less about pranking and more about quiet support; George calming Harold's anxiety before a big presentation, Harold dragging George out of a funk with a perfectly-timed callback to 'Captain Underpants.'
The darker takes fascinate me more, though. I've seen a few where one of them moves away, and the fic digs into that ache of a childhood friendship strained by distance. They're not a romantic ship for most writers—it's exploring how a friendship that was your entire world copes when life gets complicated. The best ones keep the goofy humor even in sad moments, because that's their language.
Honestly, the dynamic is so solid in canon that fanfiction often just expands the universe around them, giving them more problems to solve together. I read one where they start a community art project to save their old treehouse; it felt totally true to them.