This was originally a Twitter thread. Reposted here and edited for clarity.
40K Video Game Pitch: Whiteshield
This is a Survival Horror game. You play as the bastard son of an Imperial Guardsman. You are a 12 year old boy being raised in the unit (hence the title), as your father died in battle years ago and your mother cannot be found, and you're old enough to have started training.
This is the premise for the Tutorial Stage, where you are introduced to the game's fundamental mechanics and the variety of usable gear in the game. Aside from the standard Lasgun, you also get to see and use Autoguns, Grenades, various melee weapons, various crew-served and squad-level weapons, and you get to see--but not use--Carapace and Powered Armor when you see various elites (from Scions to a Space Marine) in the background. You also get briefed on the necessary lore that is relevant to the game here.
The narrative frame is your interrogation by the Inquisitor you saw in the Tutorial after the battle is over, with the result of this inquiry to determine what should be done with you since you faced the forces of Chaos and the Warp and survived. Multiple endings are possible, but it's going to take more than "just getting by" (i.e. a C score) to do better than get mind-wiped and condemned to a Penal Battalion, and what endings are possible are influenced by what Difficulty you played at and how well you did doing so.
Gameplay proper begins when a Chaos incursion erupts. Your unit got overrun by Chaos cultists and mutants, so you're initially trying to reach a designated point to regroup. After getting there, you encounter a Chaos Space Marine for the first time. How you handle this encounter determines your first route split choice. You receive new orders, giving you a new set of objectives to pursue in the new area you're sent to.
Gameplay mechanics include emotional control and an infection meter in addition to basic combat mechanics. If you've played a Biohazard (Resident Evil) game or the Zombie Army series, then you know the score. You may choose a difficulty level. Difficulty reflected in who is after you, not just jacking stats, with the aforementioned Chaos Marine ranging from a relatively inexperienced and unarmed one (to reflect Game Journalist Difficulty) to a Daemon-Possessed Champion (on The Emperor Protects Difficulty).
Your superiors--specifically, your Commissar--expects you to make the best use of terrain, gear, and emotional control to perform the duties assigned to you and overcome opposition along the way. You may run from everything, never fight back, do the bare-assed minimum to get out alive, and spend most of your time cowering; the forces of Chaos will tempt you in subtle ways, and the environment itself can prove hazardous if handle incautiously.
Multiple route choices are present. If you take too long to decide, then the one that is worst for your possible results at that point is foisted upon you by the Chaos Marine when he shows up. With each route choice, doors open and close--literally and figuratively--regarding other possible choices and therefore possible results. Each route choice has different Opposition Forces and they also vary by Difficulty; Game Journalist Mode is mostly a jump-scare walking simulator with stupidly-telegraphed setpiece encounters, while The Emperor Protects is the true 40k Experience that a Whiteshield fighting his way back to friendly lines could expect, and the steps between
The game ends with you facing the Chaos Marine in combat. You are not expected to win, but you can if you are on the ball and you have the means. You are expected to go the distance; the encounter is on a timer, and if you are still alive when the timer expires then the Space Marine you saw in the Tutorial arrives with his squad and rescues you.
Your score and route choices determine your ending. The best possible ending requires that you kill the Chaos Marine on The Emperor Protects Difficulty. This result gets your glorified Boy Scout recruited into the Space Marine's chapter on the spot. The worst ending, possible if you get to the end as a total coward, is that the Commissar sees that you are a coward, condemns you as such, and summarily executes you. In the middle, at either side of the line, is that you can be salvaged so you are mind-wiped as a mercy and sent to a Penal Battalion or that you have proven your worth and are awarded your unit's badge and the rank of Guardsman.
Other endings include: remanded for training as a Commissar, Tempest Scion, or Assassin; variations on your fate as a Space Marine (Servitor, Serf, Librarian, Tech Marine, etc.); reassigned to an elite unit for training; taken as an Acolyte for the Inquisitor; sent to Mars for initiation into the Adeptus Mechanicus (variations feature different sub-sets thereof); deemed unfit for duty and repurposed (after the mercy of a mindwipe) into a Servitor; and a perfect score (which includes not a single death and killing the Chaos Marine) on The Emperor Protects get you into the Grey Knights.
A playable female option swaps the Space Marine for a Sister of Battle, and all variants or interactions involving Space Marines are substituted with Sororitas equivalents.
This pitch takes its form and inspiration from a 40K story where this scenario plays out, merged with the action-survivor style of gameplay aforementioned.