It's been months since my last AU recs update, so quite a number have accumulated.
Highlander
Shades and Echoes, by auberus. Slash, Duncan/Methos. WIP
I liked the premise here that Duncan and Methos meet earlier in Methos' time in the Watchers. Not least because I like Methos & Watchers stories, and also there just aren't enough stories with Don Salzer still alive. Unfortunately this may have been abandoned because the last update was in 2009. OTOH the interval between chapter 11 and 12 was almost a year, and I've seen longer hiatuses before. Still, not the most likely candidate to be finished soon.
And Then Some, by Loch Ness. Slash and het, Duncan/Methos, Duncan/Amanda, Methos/OFC, Methos/OMC. (ca. 138,000 words)
In this AU Methos and Duncan have their first encounter during WWII.
Lord of the Rings
A Bit of Rope, by Aiwendiel. Gen. WIP (ca. 355,000 words so far)
In this AU Gandalf doesn't fall in Moria, and thsi leads the Fellowship eventually down a darker part. This surprised me a bit at first from a divergence that seems is not something going worse, yet the events are plausible and gripping.
Bond of Honor, by Meri. Slash, Éomer/Haldir. (ca. 25,500 words)
Considering that I have almost no interest in either Éomer nor Haldir I was a bit surprised that the story drew me in. But I do like the "accidental bond" trope, which these two have to cope with here as a side effect of a healing, especially when it comes with a culture clash as it does here.
Old Man Willow, by Nath. Gen. (ca. 1,200 words)
This is a short but creepy and surprisingly plausible scenario of what might have happened if Old Man Willow had been seduced by the One Ring and trapped the hobbits to get it. I really enjoyed the unusual perspective.
The Long Road Home, by Scribblesinink. Het, Boromir/OFC. (ca. 95,500 words)
This is one of the many Boromir-survives AUs, but a good one. I liked it as post Ringwar story dealing with the aftermath, and the original characters are engaging, but don't detract from the main interest in Boromir, Faramir and Aragorn.
Sanctuary
The Road Not Taken, by Penknife. Het and slash, James/John/Helen, Helen/Nikola. (ca. 4,000 words)
This has the cool premise that the energy creature possession goes differently, and John Druitt doesn't become Jack the Ripper.
Sherlock
The Republic of Heaven, by blind_author. Slash, Sherlock/John. WIP
This is meshes His Dark Materials with Sherlock, and incorporates a bit more of the HDM worldbuilding than just dæmons.
Gone Is My Past, by cyerus. Gen, John Watson, Mycroft Holmes, Sherlock Holmes. (ca. 8,000 words)
In this story John used to be a bomb detection dog who was turned into a human. I was recced this when I asked on kink-finders for stories featuring forced human-animal-hybridisation, and though this is rather the reverse, it pushed the the same kink of non-consensual body modification angst and h/c for me. And of course dog!John pushes certain D/s buttons too.
Show Me How to Stop Running, by etothepii. Slash, Sherlock/John. (ca. 12,400 words)
As anyone following my recs knows, I love werewolf AUs. In this John gets bitten by a werewolf in Afghanistan and comes back to London where he meets werewolf!Sherlock.
You Call Me Like the Moon, by etothepii. Gen, John Watson, Sherlock Holmes. (ca. 3,000 words)
Another werewolf!John AU by the same author as the one above but a different scenario, in that John was a werewolf before Afghanistan and Sherlock isn't, and worldbuilding for the lycanthropy is different too.
Star Trek Reboot
This Place at the Crossroads, by alpha_hydra. Slash and het, Kirk/Spock, Scotty/Uhura. (ca. 33,600 words)
I loved the worldbuilding and AU scenario of this AU. The premise here is that humans weren't among the founders of the Federation, and Earth's warp capability delayed compared to canon, but that Starfleet eventually sends observers, because they are close and Earth is in a contested (with non-Federation) region.
Extra-curricular Activities and its sequel
Academic Matters, by Amanda Warrington. Slash, Kirk/Spock. (both ca. 15,500 words)
In this AU Kirk and Spock meet and become a couple earlier at the Academy, and the sequel then explores how his academic hearing and the Nero events would play out.
Aurora, by kianspo. Pre-slash, Kirk/Spock. WIP
A "Spock is raised on Earth with Kirk" AU, which is a trope I love, and so far this is done well. Unfortunately it hasn't been updated recently.
Streetlight People, by ladyblahblah. Slash, Kirk/Spock. (ca. 54,400 words)
This has a great mix of angst and slowly developing friendship and romance. And I liked that this uses the prostitution trope without plodding the well worn "Pretty Woman" rescue/redemption path, even though Kirk eventually quits the (legal) sex work.
Supernatural
House of Burden, by Maygin. Gen. (ca. 76,600 words)
In this AU the fire never happened, and only Sam finds out about the supernatural because of his visions.
When the Badger Grows Horns, by San Antonio Rose. Gen. (ca. 25,800 words)
The premise of this AU is "what if John Winchester was Diné?" and what made this most interesting for me was to see Sam and Dean with more extended family to fall back on and fewer rifts between them.
White Collar
Always Starts the Same, with a Boy and a Girl, by lightgetsin. Slash and het, Neal/Peter/Elizabeth. (ca. 21,500 words)
In this AU Elizabeth met Neal first rather than Peter, and things turn out differently for them. This quite charming and had a nice symmetry to canon without being too contrived.
X-Men: First Class
Limited Release, by Anon. Slash, Charles/Erik, Alex/Hank. WIP
This is a fusion with White Collar in which Erik is a mutant in the FBI and Alex is a mutant criminal, released to help him hunt for Shaw. It doesn't try to match up with the White Collar scenario completely, because it is very much about mutants rather than con artists, but the world building is great.
Any Measure of Peace, by manic_intent. Slash, Charles/Erik. (ca. 19,400 words)
The beach scene at the end goes differently in that the Americans and Soviets do not try to blow up the mutants. I enjoyed how it introduced Fury and Weapon X here as government factions into the plot, and how Charles injury happened later on.
Time to Grow, by zarah5. Slash, Charles/Erik. (ca. 21,000 words)
This is another fix-it for the ending of the XMFC movie, in that Erik sends Azazel back to evacuate Charles to a hospital which leads to a better outcome of the injury and eventually also gives more opportunity for reconciliation between Charles and Erik. And I really enjoyed the latter aspect of the fix-it, and how they mend ideological fences, but I'm somewhat ambiguous that it also erased Charles disability at the same time.