It's a shame that Dead of Winter is focused on the Arctic conditions, because there's an interesting zombie replacement: Snowmen. You could go silly or scary with it. They'd still be shambling and slow, and you'd never know when one would form. You could even have it be a nuclear winter and radiation cause it.
Or maybe it's a terrible idea. Plus, it'd make it too close to Dead of Winter, thematically.
Maybe you could go with parasites like the ear worm things from Star Trek 2. Where the "others" appear to be normal but are acting strangely. And they could even be fighting against it, providing you with the seeming stupidity and slow movement of a zombie.
Or go Cthulhu without going Cthulhu. Have them be an extra-dimensional invasion, but coming from somewhere with much lower pressure, so they have to practically swim through the air here. Would be interesting because you could do a mountain scenario which would be much harder in the thin air, speeding them up & slowing the humans down.
*Shrug* Just jumping for some off-the-wall ideas. And probably none of these would fit with your game idea.
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