Plot
1943.
Ozwell Spencer and Edward Ashford have recently discovered the "mother virus," and have founded the small pharmecutical corporation Umbrella to provide a cover for their research. They're both rich men, but the company needs to start turning a profit.
Fortunately, there's a war on, and the American government is being generous with its research funding. Spencer and Ashford spend some time weaponizing the "mother virus," and then make a few phone calls.
1944.
As the Allies make a drive towards Berlin, they hit scattered pockets of resistance. The German people are tired and ready for the war to be over, but retreating soldiers and die-hard civilians make things difficult.
One of the toughest nuts to crack is a small, ancient castle in the countryside. Several dozen soldiers have holed up inside it with ammunition and food, and the castle has its own water supply. They control the road leading up to the castle, and have the high ground. They could hold out for years. This is unacceptable.
The decision is made to bomb the castle with Spencer and Ashford's new weapon.
Several days later, behind enemy lines with a broken radio, a platoon of tired American soldiers comes across the village. Exhausted and hungry, they figure they've found an abandoned village. They don't realize what's actually happened, until it's too late.
Setting
A German village (probably a fictional one) with an old castle attached to it. The "mother virus" has been released, turning the German soldiers inside the castle into zombies, or possibly something worse.
Monsters
The "zombies" we're used to in RE are usually made from the T-Virus; canonically, we've never seen what happens if somebody's been infected by the "mother virus." There's a lot of room for creative license.
The virus may have also infected dogs, cats, farm animals, plants, earthworms, spiders, or more. Since the castle obviously isn't concealing a lab or anything, the monster population will be all "accidental"; no Tyrants, no Hunters.
If a "boss creature" is called for, I was thinking about lifting the idea of the overfed "super zombie" from an online novel called Zombie Nation, which is well worth reading.
Characters
American GIs separated from the main drive into Berlin, presumably all members of the same unit and probably all veterans of D-Day. You could also have British or French troops who're in the village for basically the same reason; paratroopers who went off-course and are looking for someplace to get their bearings from, angry members of the French resistance...
German survivors, either soldiers or civilians, either friend or foe. A scheming German soldier who survived the gas drop would make a good sort of bastard.
As far as characters' basic stories and whatnot go, I'm thinking more Saving Private Ryan than Sgt. Rock. Think more everymen thrust into a terrifying situation than a cigar-chomping hardass.
Mood, Theme
I'm thinking of this as sort of like Indiana Jones & The Call of Cthulhu. Scary, big explosions, big action stunts, PC death, high creepy factor. Bashing Nazi zombies, man; come on, that's classic.
Players
Anyone. You don't need any RE canon knowledge to play this game, and all characters will necessarily be OCs; even the RE canon cast members who are alive at this time are cast members who have never gotten a line or been onscreen over the course of the series. Surviving characters may be worked back into a modern plotline.
When
I'd discussed the idea with Dave of doing this to have fun while our primary RE cast were training together to be the new STARS team. So basically, we can throw this together whenever we have the time.