A lot of good options here.
A Toby Fox thread would be difficult imo, since he has pretty good OPsec and tends to stay out of the public eye.
A dive into /v/ would be basically be the opposite side of the Tumblr coin, and I think it would be really interesting to see how many recurring characters from the Tumblr Files pop up again, considering how many tripfags probably ended up becoming funnymen in the long run.
KYM is a site I haven't thought about going into like that, to be honest. Most of the time I just go there when a new stupid meme (i.e. 67) pops up, and usually I'm only there to figure out "why" this onslaught of shitty memes keeps getting worse. It's a site I've mostly ignored, so if there's something juicy there I'd be interested to see it.
A history of Sonic lolcows kinda feels poetic in a way. That fanbase has been fertile ground for spawning cows since even before a certain Virginia resident won a certain sweepstakes. I'd like to try to get to the bottom of why so many autistics latch onto Sonic in the first place, since such a high concentration of autism in one fanbase can't be coincidental. There's gotta be some kind of pheromone in those games that captures them in such magnitude.
Triple-Q seems like one of the biggest single lynchpins in all four of your previous cases. Whenever you'd drop a bombshell, it seemed like he was always right at ground zero. How many heads does that hydra really have? I was so close to voting for him, but there was one other option that felt more compelling to me.
Tumblr's retro game obsession would kind of tie into all of the aforementioned areas in one way or another. Most of Tumblr's retro game fans probably came from /v/ or Something Awful. Toby Fox's background in the Earthbound hacking community directly inspired Undertale's artstyle and gameplay. Emulators like Dolphin led to a re-evaluation of games like Wind Waker and Mario Sunshine; and I have no doubt emulation led to a similar appreciation for Sonic Adventure that we see today. Retro game OSTs were the bread and butter of Soundclown. Big YouTubers like James Rolfe built their careers off of mocking "shitty games" of the past, and exposing them to a whole new audience. A lot of the people in those spheres would go on to
make games of their own, all using the same crunchy, low-quality PSX/N64/Dreamcast
"A E S T H E T I C ®".
I feel like this would probably be the best topic to get into, since it seems like almost everything else seems to stem from this nostalgia-obsessed corner of the internet, especially the retro game fandom.
