The poll has spoken!
Because this one-shot is going to be based on Long War instead of Enemy Unknown / Enemy Within, you can expect some major differences to crop up. Of them, I've compiled a list of the most significant. They are as follows...
In this AU S.H.I.V.s serve as the precursor to the GREMLINs that Specialists in XCOM 2 utilize. So if you choose to play as an Engineer, then you will automatically be eligible to receive a S.H.I.V. of your very own to command!
XCOM soldiers in this AU do not need to receive injections of Meld along with Base Augments in order to operate a Mechanized Exoskeletal Cybersuit, only training. However, pieces of the Base Augment set are available to soldiers who've lost one or more limbs in battle against the invaders, along with more advanced pieces that're reminiscent of the augmentations available to people in the Deus Ex and Metal Gear universes.
In this AU all MECs are capable of acting independently of their pilot thanks to a Guardian Chip a la TitanFall. When a MEC Trooper exits his or her MEC and remotely activates the Guardian Chip via voice command, the hole where the pilot's head and neck would poke through immediately seals itself shut. Much like a dog, the "Auto-MEC" only responds to very basic commands -- stay, guard, follow, attack, etc.
(Note: Long War lore dictates that MECs and MEC Troopers appeared much later in the campaign compared to vanilla due to the need for more time, resources and research required to build them. That being said, since there are multiple XCOM bases in operation around the globe in my story, I believe it's safe to assume that one or more of them would have produced at least one functioning MEC before things went tits up.)
XCOM 2's story tells us that XCOM was defeated in the early stages of the war against the aliens and never reached any technological progression beyond conventional weaponry, meaning that canonically the Volunteer never existed and there were never any troops with unlocked Psionic abilities. I won't be adhering to this piece of lore, but I won't be allowing any of you to play as a Psi Operative either. In this AU only five "gifted" humans existed prior to the events of XCOM 2, namely Anna Sing, Anette Durand and the three Furies; Mattie "Alecto" Hawkins, Nubia "Megaera" Tariq and Fatima "Tisiphone" Tariq.
In my mind the main source of Meld came from the bodies of the alien corpses that XCOM recovered for study. This naturally made a lot of people leery about participating in the Meld Recombination research project. However this changed after several containers of the substance were discovered aboard a crashed Supply Barge.
(Note: Long War lore dictates that installing one or more Gene Mods requires an extensive amount of time spent in the infirmary. So, for the sake of keeping things balanced, I'm going to be saying that only a handful of genetically modified operatives were produced before XCOM's fall.)
While not a really a "big" change, it should be worth noting that people who have been installed with Gene Mods are no longer identified by odd plugs in their arms, but rather veins that become bioluminescent whenever a certain ability is activated. For instance, a soldier making use of Hyper Reactive Pupils would look something like this.
The Chryssalids of my AU are very different from the creatures you're all familiar with. They begin life as a Silacoid that a matured Chryssalid creates via regurgitation. While in this larval stage, the creature is compelled to seek out a host body right away and enter it by any means possible. The body in question need not be alive, although a living body is preferable since it will transform into a Sleepwalker only minutes after being possessed. As a Sleepwalker, the host will do everything it can to ensure that the creature growing inside of it reaches maturity. This can be anything from running away and finding a place to hide, to taking up arms against people it perceives as a threat. No matter the case, unless the Sleepwalker is dispatched in a timely matter, then in as little as 30 minutes a Chryssalid will emerge from their body almost fully formed!
In this AU all Sectopods are the same size as the one revealed during the end of Enemy Unknown's Developer Walkthrough.
In this AU the unit in question was not mass-produced. Instead only one was created, and it was his job to see to it that not only was EXALT created, but that it's pro-alien agenda was spread to the masses. (...)
In this AU the EXALT organization was not only clearly the precursor to the ADVENT organization from XCOM 2, but also the driving force behind the shadowy Cult of Sirius. It's also worth noting that this incarnation of the organization had no qualms about inducting women into their ranks, and soldiers dressed more like Militia than businessmen.
In canon the Avenger is a converted alien supply craft. In this AU ships of this type were used exclusively by the Ethereals. For the sake of keeping things open-ended, I'm going to go ahead and say that the Avenger used to belong to the so-called "Overseer."