A cosmic horror game by Bethesda using one of the Elder Scrolls engines. Giant monsters, creeping things, things that don't die when you stab them, tentacles, all that fun stuff. Plus open world, lots of easy modability, and bugs that might help or hinder the player, or make them laugh or freak out. They could even add in a few things that seem like bugs but are there intentionally like Eternal Darkness did. Textures tearing, things shooting off at in a random direction, and NPCs that fall into the depths of the uncanny valley could add to the atmosphere.
The setting would either be modern times with magic and Things-Man-Was-Not-Meant-To-Know coming back or after an apocalypse caused by the afore mentioned event. I'm a bit torn between wanting magic and technology to work together (lots more possible bugs) or for them to cancel each other out (like in Arcanum). Either way there would be lots of magic arms and armor and lots of firearms.
Firearms would be like in Fallout 3/NV but with the scale going higher so things like
this would be used at higher levels instead of the puny .50BMG. Some relatively common monsters would be big enough to need that much firepower.
Magic would cost sanity to use. Low sanity would cause hallucinations like seeing something that's not there, seeing things as something they're not, and animation failures.
There would be some monsters that would seem unbeatable. Extremely fast regeneration, high hit points, tough armor, or a combination of those would make defeating them seem impossible. True cosmic horrors that can only be run from. Players will probably find bugs or make mods that let them beat the impossible horrors but that's fine. It fits perfectly with the theme for players to defeat them using something else that should not be.