As preparation for my imminent return to writing WITWICS fics, I've been going over my headcanons and timeline. Here are some things that will be relevant to the fics I've written/will write--
* All the fics I have written, h/c-filled or otherwise (the really h/c-heavy ones are locked as friends-only, regardless, though since most of them were cross-posted to various comms you can probably find them publicly if you look really, really hard for them. But you're on your own for that), are part of the timeline, with the following exceptions: "Home by the Sea" is 100% retconned and I want to forget that I ever wrote that thing; "Second Home by the Sea" is NOT retconned; it is still part of the timeline, but has been edited to eliminate all reference to "Home by the Sea" (aside from the title, which... is going to be a relic title since it no longer makes sense, heh).
You can read the edited version here. The other pieces to be retconned are anything referencing the continued marital pursuit of the ACME Messenger by Aranea Vulsor after their initial interaction; as ACME no longer has a role in the Anubis fiasco, Vulsor's chasing after the Messenger makes no sense now--she may have wanted to continue pursuing him, but the Anubis thing and Sebastian Moran asking her to do a favor (in one of my BBC Sherlock fics) will have drawn her attention away from him.
EDIT: I've decided that I'm also retconning the Informant's initial distrust of the Messenger during the beginnings of their time together; rather, I think it'll be retconned to the Informant being more cautious at first around him, but then quickly warming up.
* After a string of back luck while tailing VILE agents, the Informant has learned to adapt to VILE's methods and is better at following them without being spotted. This is my in-universe explanation for the lack of Dying Informant sketches in the show post S3, as well as the lack of h/c for any future fics.
* My version of the Informant is a composite/combo of the Dying/Bouncing Informant and the Canine Language Interpreter characters; he still works with Fido, but his canine companion of choice is Borealis, a rambunctious Siberian husky. The Informant also sometimes helps out with his friends' departments, like Tech-Net and Fresh-Net, if he doesn't have any field assignments.
* Borealis has a problem with getting along with cats. His archnemesis is Mrs. Pumpkinclanger's Siamese cat, Godiva (her cat made an appearance in one sketch, albeit unnamed).
* The Messenger keeps any tips/gratuities in a petty cash fishbowl that any of his close friends can access if they need it.
* The Informant works best with the Messenger, the Techie, and the Inspector because they function as a unit. The Inspector has the best observational skills; the Messenger has the best leadership/planning skills, the Informant has the best action abilities/general knowledge skills, and the Techie has the best technical knowledge/practical skills.
* Mr. Schwemphf is still the vice-Chief who is an extreme stickler for rules and is overbearing and too strict. No one likes him, but the Chief can't fire him because he hasn't done anything wrong. So she settles for looking the other way when the Messenger pranks him. This happens a lot.
* Conrad Knuckle and EMCA are still bad guys in my 'verse. They turned to wrong methods to try to bring VILE to justice, and though they were accepted as "good guys" in their S2 introduction, they no longer are now. EMCA has a deep grudge against ACME for refusing to help them and their wicked ways.
* Carmen Sandiego is the daughter of a South American businessman, Malcolm Avalon, who disowned her because of her thieving ways; that being said, she still loves him and will do anything to protect him (this is Earth cartoon's canon, but I liked the idea and am borrowing it for World). Carmen's sister--Patty Larceny's mother--is in contact with both Carmen and their father, sometimes acting as a go-between.
* Sarah Nade has an older brother named Fanfaro Nade. He is not a member of VILE, but sometimes does stuff for them to help Sarah out.
* Aside from the rooms we're shown in the show, the ACME building also has dorms, a cafeteria, a lounge, a library with an archive room, the Tech-Net lab, the courier depot, two computer labs (one attached to the library for general agent use and one attached to Tech-Net), a rec room, and the Ruff-Net kennel. There is also the health food shop area of the building, which serves as the front.
* The Informant is a bit of a neat freak.
* Animals are naturally drawn to the Techie and absolutely adore him
* All of the plumbing in ACME is done by the Mario Bros. I have no reason for this other than the fact that I love having my fandoms mesh. :)
* Speaking of fandom meshing, the ACME crew have heard of the Doctor. Some of their European cases have involved them with UNIT, and the Techie has occasional professional contact with Kate Stewart, the Brigadier's daughter, as she is currently the head of scientific research. In addition to hearing about the Doctor from UNIT, ACME has often heard Kneemoi singing the Doctor's praises.
* ACME Time-Net exists, but it exists as a part of the main ACME Crimenet, as opposed to being a separate entity like in "Where in Time." Specifically, it's an offshoot of the Tech-Net department, and the Techie sometimes works on the quantum physics part of the research there, as well as working on the development of the Chronoskimmer--development that would get a significant boost upon a visit to ACME by the Doctor (cueing some puzzled reactions from the agents who were not expecting a total dork since Kneemoi was going on and on about "The Oncoming Storm")
* For the record, "Where in Time" is not a part of my timeline; Carmen is not as evil in World as she is in Time (in Time, she seemed to really want to create purposefully harmful time paradoxes and was different than her World counterpart, where she was stealing things for the thrill/challenge/fame/wealth) and would only use time travel for the same reasons she took things in World. The Chief is staying at ACME Crimenet, and any usage of the Chronoskimmer will be at the hands of the Techie (and whether or not there will be any time travel remains to be seen)
...That's everything I can think of for now; I'll add other things if I think of anything!