Well I certainly dont think Gosho should go dark and edgy for the sake of being dark and edgy. When I said I thought Gosho originally planned Gin/Sherry backstory, I was expecting the same standard childhood romance:
Gin and Haibs met in America when they were both young (they were both born into BO families perhaps) and Gin was still an idealist who doesnt believe in violence, etc. Then one day Akai Sr or Mary killed Gins dad/mom/best friend/cousin/etc so Gin started hating FBI and the extended Sera family including Haibara and Shuichi and turned quickly became some top tier evil BO exec.
Pretty cheesy but standard Gosho.
I wouldn't call that Gosho's standard romance. Just them being BO and how it eventually turned out (an outcome that allows no happy end) makes it already way different and darker.
Then there's also the striking age difference. The longer any closer history between them dates back, the bolder it gets. (But then again, we don't know Gin’s age. My guess is that he's about 10 years older than Sherry – that would make him about 28 years old now, but maybe he's younger?)
But yeah, I could totally see something along the lines of that they've known each other for a long time and that he used to be different (I mean at some point in his life he must have had some more humanity). Add to that how being born and raised in the BO distorted Sherry's judgment plus maybe some daddy issues given that she's an orphan who never got to know her parents – et voilà, this pairing makes sense.
I don't think he could hate her for any Akai-family-relation, because it seems like Shu and Sherry don't even know themselves that they're related, but I can see that you're just being hypothetical with this.
Long story short, I think Gin/Sherry would still be a bolder move on Gosho's part (considering DC standards and how other love stories/couples are like).
Either way, Gosho has revealed nothing about Gin's past, so I'm wondering if he's saving this for some special reason. Like you said, Gosho exploring Gin's past will definitely be a pretty big deal. I'm expecting him to go into it after Rum's arc finishes.
Now that I think about it, some mysterious history with Sherry is literally the only tidbit that has ever been teased about Gin's past, that very well could speak volumes about the significance of that possible pairing. No matter if their backstories are deeply tied or not, I'm really looking forward to finally learn more about him and it would be great if we really get to that as soon as the Rum arc finishes.