For all their achievements in narrative design and experimental, interactive storytelling, sometimes a fan theory is just... better. It can explain where a forgotten character disappeared to, how longstanding rivalries actually came about, or why a given plot hole isn't really a plot hole at all.
It can take a character so barely fleshed out as Boba Fett and make him legend. In short, fan theories are the continuation of any property's narrative, except instead of only a handful of writers, we've got the entire globe's populace working for the rest of time.
Pretty cool, right?
Thankfully, we nerds of the world have a tendency to overthink things more than anyone else, resulting in all manner of twists and reworked ideals when it comes to - in this case - gaming's biggest properties.
They range from dark to delightful, and I'm starting with...
Okay, this one is dark, but don't worry, it isn't a 'start how you mean to go on'-type deal (OR IS IT?!).
The theory goes that in Arkham City, when Harley first appears and fights Batman, she was pregnant - as is proven by easter eggs in the main game, and the Harley Quinn's Revenge DLC. In the campaign, Quinn immediately lunges at Batman when he comes across her, only for Bruce to violently counter and slam her down directly onto her belly, where she then lies in pain for quite some time.
(3:06 on the below video)
The Revenge DLC supports this, showing the Scarface doll all made up in a crib to represent a 'child Joker', as many now-negative pregnancy tests litter the room. In the standard canon, it all tends to be framed as a combination of Joker's death and the aforementioned 'false positive' providing motive for Harley's state of mind, but factor in this theory, and Gotham just got a whole lot darker...