Ferrous dragons appeared in dragon magazine, & got updated, with sample dragons, for 3.5. Fivnikama is the nickel dragon, I started with him since he was the weakest sample dragon. When he died his heart turned to a lump of nickel worth 8000 nickels- 400gp. Then other ferrous dragons showed up, 1 per week, to avenge their kin killed the week before. I stopped when I had 2 Iron Dragons left, who were way beyond the party's level, but they'll show up to pick a 3-way fight between the party and a red dragon. One of my favorite strategic elements is "we're fighting the same enemy but we're not on your side". Dragons are a little more dangerous on my world- "awaken SR" feat from Dragonomicon gives them SR from birth, and Psionic resistance at -5, = amounts for psionic dragons. Same for fear aura. Even a cute, little bitty dragon is recognized by all life forms as the apex predator that will grow up to be a threat one day. Dragons all have Power attack and take -1 to hit for +1 damage, per age category. The +60 to 72 damage per round from a Great Wyrm who won't miss even with the -12 attack lets the PCs know they're in an escape & evade situation. DR is +2 per age, maxing out at 25 for Great Wyrms, 1/2 of that vs. dragonbone or dragonbane weapons (for the wrong color- using black dragonbane vs. red) and 0 DR vs true dragonbanes or holy weapons, or vs. barbarian rage.