When I was reading up on metamagic feats for familiars (see responses here: http://skirmisher.com/node/956 ) I suddenly got an idea for a dragon using extensive meta breath weapon feats, a result of them using their intelligence as well as magical might. These were introduced in Dragon 296, updated in the 3e Draconomicon and there are some variants for Pathfinder in Novus Draco from Ennead Games (I never owned Dragon Magic so I have no idea if there are any in it). The feats increase the amount of down time between breath weapon uses. I give some suggestions for these duration bumps (listed like spell bumps for metamagic feats) and you should modify them to your needs. Now if you use these ideas, do not blame me if the players throw objects of various sizes and density at you. I wasn't joking with the post title.

 

Invisible Breath Weapon (+3) is the most dangerous to opponents. Combined with Improved Invisibility on the dragon and the PCs may never know what killed them. Even if the PCs are immune to the energy type of the breath weapon, it is doubtful their surroundings are. And it not only makes the breath weapon invisible, it is also silent and odorless.

 

Concussive is one I mentioned in the deepspawn post (+2). It causes everything that fails their save versus the breath weapon to be thrown back 10' per caster level of the dragon and they take falling damage. A +4 version causes knockback to those who are immune to the energy type of the breath weapon. In any case, most dragons will fling characters so far it will take them a few rounds to get back into melee (though who in their right mind goes toe to toe with an elder dragon?).

 

Alter Element (+1) allows the dragon to select the energy type of its breath weapon when it makes the attack (thus +1 versus the +0 of the metamagic feat). If the PCs are obviously immune to its initial attack, the dragon can adapt. A +2 version allows the dragon to pick a lesser known energy type or something like holy/unholy.

 

Selective Breath Weapon (+2) allows the dragon to target specific materials with its attack. A +3 version is needed for acid and sonic to prevent these from melting or shattering non-target materials. The dragon can destroy flesh and leave the loot safe so it can be added to the monster's hoard.

 

Destructive Breath Weapon (+3) is an improved version of Selective and causes the targeted material to explode, doing an additional 50% of the damage to everything within 10' (or more with larger dragons).

 

Refracted Breath Weapon (+1) is only useable on lines. It causes them to bend in midair up to a 90 degree angle.

 

Combining some of these will make the dragon very lethal. An invisible dragon with a hidden breath weapon that automatically causes knockback to those who fail their save and causes steel objects to explode has to wait for d4 +10 rounds before it can use the attack again. Of course it is doubtful the PCs will want to fight after their weapons and armor is destroyed and are waiting in terror for the wizard to cast Teleport to get them the hell out of there. Don't dragons have access to Dimentional Anchor? A smart dragon played intelligently should lose only when the players are at their best.