DC Universe

The Mystery of the Bat, by Basingstoke. Gen. (ca. 3,300 words)
In this Elseworld the Bat vanished a few years ago, but the public didn't notice because Barbara Gordon picked up his role, aided by her adoptive brother Dick and later by her sidekick the Fox. Gotham's current vigilantes never met their predecessor personally, however now they are going to solve the mystery of his disappearance and identity.

Family Business: Five Things that Never Happened to Dick Grayson, by Cereta. (ca. 6,600 words)
A series of five vignettes, each an intriguing Elseworld scenario. My only "complaint" is that I wish there was a longer story set in some of the universes, because the glimpses whetted my appetite for some of those.

Hanging Work, by Cereta. Slash, Bruce/Dick. (ca. 44,800 words)
This expands one of the snippets from her Five Things story into a fully fleshed out AU series. It's the scenario in which Dick Grayson is eventually taken in by Selina after the death of his parents and becomes a cat burglar.

Cross Fade, by Cereta. Gen, Bruce, Dick. (8K)
A sequel to Lights Going Down, another of the Five Things scenarios in which Bruce gives up being Batman after Dick is seriously hurt. In the sequel Dick picks up the mantle of Batman and Bruce finds out.

The Nature of Family, by Ethrosdemon. Gen.
Kal-El from a different timeline, one in which he had grown up as Bruce's child and Dick's brother, meets the Batfamily of the main DCU.

Fearful Symmetry, by Kerithwyn Jade. Slash, Batman/Harvey Dent. The story is finished, but the universe is still being expanded as a WIP.
What if Bruce had stopped the acid attack on Harvey Dent in the courtroom? 'rith also posted a cool timeline for this AU, as well as a series of so far three drabbles, all of which are linked from the story page. (also on on AO3)

The Call, by Marcelo. Gen, Bruce Wayne. (ca. 450 words)
A short, yet scary DC AU. The author describes it as "DC Comics meets Lovecraft" and I really like the creepy premise for Batman here.

Mobius Paths, by Marcelo. Gen. (ca. 300 words)
This is short, but with a very neat though somewhat creepy premise: What if Alfred Pennyworth was really J'onn?

Shards of Night, by Marcelo. Het, Lois/Clark. (ca. 830 words)
Clark in this story is more alien than in canon. The story is on the scary side, but I actually felt sorry for Clark, because he's still one of the good guys.

Topsy Turvy World, by Roach. Gen.
This is a response to the Vice Versa Challenge and swaps Tim and Barbara, that is Tim is shot by the Joker, who targets him after he notices that Tim is stalking the Batfamily, and Tim ends up as Oracle.

The Road Not Taken, by Slipstream. Gen, mostly Jason and Tim.(ca. 32K)
What if Jason had survived the Joker's attack and the explosion, but was injured?

Intimates Series, by Te. Slash and Het, Tim/Babs, Tim/Jason, Jason/Dick, Jason/Steph. (The series is still ongoing, but each story is complete.)
Basically Tim works for Oracle as an operative in Gotham, eventually is discovered by Jason (who survived in this AU) who's still working with Batman, but it is so much more than that. The way Tim and Barbara interact with each other and their selves/bodies, both in real and virtual space is fascinating, especially how they change and sometimes blend into each other because of how they use technology and fantasies. Not to mention that I also find it really hot. I also liked the tension between Barbara and Bruce and their claims on Gotham's vigilante space, and how Jason reacts to what being Avatar entails for Tim.

The Compass That You Gave Me and its sequel Take The Black Off A Crow, by Zee. Gen, Dick, Kon, Clark. (ca. 19,300 and 3,600 words)
What if Dick had found Superboy after he escaped from Cadmus Labs and taken him in?