Stargate: AtlantisEverything But Love, by Cottontail. Slash, John Sheppard/Rodney McKay. (ca. 24,200 words)
This story adapts the plot of the movie
The Lake House. I have never seen that, but the crux of it is that John and Rodney get each others' mail, but are living two years apart. The time travel paradoxes in this one made my head hurt a bit, but then the time travel here is more a vehicle for their romance than the point of the story, and the romance was a lot of fun to read.
East of The Sun, West of The Moon, by Dossier. Gen, John Sheppard, ensemble (there are some het relationships though they aren't the focus). (ca. 150,000 words)
This is simply an awesome SGA novel. The premise is that they don't quite have as much luck in the episode The Intruder, and the Daedalus is shipwrecked on a planet. It is John-centric in its POV, though it is very much an ensemble story, and the worldbuilding is fantastic. The alien culture is interesting and complex, the original characters are real people without taking away too much attention from our heroes, and it features Pegasus galaxy history and Ancient tech as background to the plot of the stranded Daedalus too.
At The Hour When We Are Trembling, by Frostfire. Slash, John Sheppard/Daniel Jackson. (ca. 36,000 words)
In this post-apocalyptic AU the Wraith make it to Earth, and John and Daniel lead a resistance cell. It's very griping, I couldn't put the story down once I started.
Not in the Stars, by greenconverses. Gen, John Sheppard, Teyla Emmagan. (ca. 3,900 words)
In this AU John is an Athosian and Teyla earth military. The story is mostly set pre-series, explores both their lives, and then ends with their first meeting.
Second Best, by Madison. Slash, John Sheppard/Rodney McKay. (ca. 29,800 words)
It's a quantum mirror story in which Rodney, after John's death, tries to fetch a John from another universe, who happens to be a down on his luck gambler. This was a solid action-adventure, with some angst thrown in, as Rodney and the team still miss the "old" Sheppard while getting to know his alternate universe version, while John has to cope with being a replacement.
The Protection Racket, by Pares. Slash, John Sheppard/Rodney McKay. (ca. 14,000 words)
This story is great comedy, and made me laugh a lot. Things are set in motion when Jack surprises Rodney as he plays strip poker with Sam, causing Rodney to flee, freak out, and fear for his life, and then to hire John as his bodyguard to protect him from Jack. Only professional bodyguards are harder to find on short notice than Rodney had hoped, so he hires John based on an ad in miscellaneous "Any Job. Sliding scale." Hilarious hijinks ensue.
A Rough Road Leads to the Stars, by Sonadorita. Slash and het, John Sheppard/Rodney McKay, Ronon Dex/Teyla Emmagan. (ca. 7,240 words)
It's an SGA version of the Apollo 13 mission, called Atlantis 13 here: Sheppard, Ronon and Teyla are the crew and Rodney coaches them from the ground. I found the choice to set it in 1967 instead of in the year of the actual mission a bit odd, but apparently the prompt it was written for asked for the 1960s, hence the random seeming timeline shift. But I enjoyed it otherwise.
City on the Edge of Forever by synecdochic. Gen, Elizabeth Weir, John Sheppard, Rodney McKay, Aiden Ford. (ca. 6,650 words)
I have no idea why I haven't recced this previously, since I first read it some time ago, but recently I reread it, and noticed that it wasn't on my AU recs page. So it's quite likely you've already read it as well, but if not, it's an AU retelling of the pilot Rising, close to canon but with a more cunning expedition, who come prepared for the worst, and make smarter, more tactical decisions. It's also a cool take on how Elizabeth could have been given more room as an expedition leader.
The Lonely Sea, by Tarlan. Slash, John Sheppard/Rodney McKay, Jack O'Neill. (ca. 26,500 words)
This is a fusion between the Stargate universe (both SGA and SG-1) and Waterworld, i.e. an Ori attack caused the polar caps to melt. I loved the worldbuilding and the way the two were merged. I cared a bit less about the John/Rodney romance relationship here, but the worldbuilding and rather cool action-adventure plot made more than up for it.
SupernaturalThis Wheel's On Fire, by Marin Rusalka. Gen, Dean, Bobby, Sam, Jess. (ca. 27,800 words)
This is a fusion with the movie
Ghostrider. I haven't watched that, and don't think familiarity is necessary as the basic explanations of the Ghostrider premise were covered and merged with SPN mythology too. In any case the image of the flaming Impala leaving blazing tire tracks is better than a flaming motorcycle, and I loved Dean's conflicted reaction to being a Ghostrider.
Rain Fall Down, by Nilchance. Gen, Dean, Bobby. (ca. 2,185 words)
It's the third part in Nilchance's
series, and I've already recced the previous stories in this universe, but I thought I'd point it out again, in case you missed it, since it's been a while since the last part. This is a rather bleak AU scenario that explores what might have happened had Mike Gunther called Child Protective Services.