Yeah, I don't know how accurate this theory is.
Amanda died fighting Vampires in a hell dimension.
Molly was murdered by a super-powered madman.
Jonathan was sacrificed.
Warren was flayed alive by a witch.
Then there's Buffy. Are you going back as far as The Gift, or would only the being shot to death in Season 6 apply? If the Gift doesn't apply for Buffy, you can't include Joyce in your list of natural deaths.
Off the top of my head the only characters I can think of who died from what you would call natural causes, would be Joyce, possibly Tara (if murder can be called natural), and Cassie Newton.
I'm not even sure Anya would apply, unless being cut in half by an eyeless cultist in a battle to ward off an apocalypse can be considered a natural cause. Hmmm... maybe in the Buffyverse.