An open world Warhammer 40K Imperial Guard game, playing mainly like a survival horror. You are a single imperial guardsman alone on the front lines of a vicious warzone, and have to make your way to a HQ a huge distance away to get airlifted, crossing randomly generated battles between allies/enemies, all of which could gut you in a second if you catch their attention.
The journey would take several days, and be completely unscripted, allowing virtually all the coding being focused on creating a 100% natural and logical interactions between all the random units on the battlefield.
Each unit would have a dozen variations of weapons, colour scheme, numbers, fighting ability etc. so that you'd always get a completely different experience.
Just imagine flitting from cover to cover as an emperor titan towers overhead, shaking the earth and knocking you over with every step, or trying to run away from a brigade of Ork troops, charging your position. Taking cover from a Tau artillery barrage that rips up the fully destructible terrain, or tagging along underneath a Leman Russ tank as it happens to be going in the same direction as you, staying near it from protection. Seeing a Space Marine squad launch into the fry and decimate everything near them, before charging off at too high a pace to follow leaving devastation.
It's not even that far-fetched, when you think of what Skyrim can achieve as regards the size of an open world with no loading screens, and the scale of madness some of its mods are able to achieve by creating randomly generated battles in the world, Now reskin the models and units to be 40k related, strip away virtually all quests, dungeons, cities, skills, crafting, armour, items, and use those extra resources to increase and improve the random generation system and uniqueness of units.
That would be ace.