Has anyone tried a variant whereby instead of creatures having DR vs a special material e.g. cold iron, that said creatures instead have no DR but take extra damage from the substance?
Can't say I have, but it sounds like a good houserule. It fits the classic "The silver candlestick seemingly burns the werewolf's flesh." image. I'd say something like 1.5 times the normal damage dealt for the special material.
However, I'd pad the monster's hit point significantly. If, with DR, the fighter with a normal longsword deals 2 points of damage and the rogue with a silver short sword deals 4 point of damage to a werewolf, the same werewolf would take 7 from the longsword and 6 from the short sword under this system. Game breaking? No, but the combats would be significantly shorter.
However, what would you do in the case of damage type AND material based DR, such as the bludgeoning and magic DR of a lich?
Has anyone tried a variant whereby instead of creatures having DR vs a special material e.g. cold iron, that said creatures instead have no DR but take extra damage from the substance?
I think there's a Dragon issue that had something like that. Can't remember the number, but it was the one with The Plunderers Handbook and the Pole Arms.
If you want a real dramatic effect you can always use both.
If a were-ooze normally receives a DR 5/silver, you can also grant silver objects and weapons a bonus x1.5, x2 or even x3 damage.
This works well for custom monsters. Especially when their only weakness is supposed to be (insert weakness here).
Good question... I don't really like "damage type" DR so I would maybe ditch it. But that would also mean more hps probably :D
Maybe a good guide line for increasing HP (if you changed DR against X to a weakness against X) would be +50% for DR 5, +100% for DR 10 and +150% for DR 15.
This seems like a large boost but it really depends on how much of a weakness they have to the material/weapon type. I think a normal boost would be double damage. Alternates could also be automatic crticals or a different damage boost level.
I was thinking double damage, and I like your scaling of hitpoint increase.
Found it! Dragon Issue #331!!!