Dragon breath weapons are boring. Scary, but boring. So here are some ideas for breath weapons of dragons and other monsters. There are three catagories:

Teleportation. The breath weapon teleports the PCs, their bodies only, their gear only or a preselected object(s). For the last one, this is generally a boulder or wall of iron teleported above the targets so they will be squashed. Dragons don't like doing this because it destroys potential treasure but other monsters are just fine with it. As for teleporting gear, most dragons are smart enough to send the gear to a holding area so that they can look through it before adding choice bits to their hoard. To many hidden parties in portable holes and lost treasure or lives. And then there are those monsters who teleport only certain materials. An ethereal filcher that has a breath weapon that teleports gold or gems may not even be seen as a threat until the party tries to buy something. 

Summoning. For dragons elementals are more common than outsiders. A red dragon's breath may summon 1 hit die of fire elemental per X hit dice of dragon. The summoning is permanent, though the dragon can banish the creatures at will. For all monsters, they may summon any sort of creature. And summoning doesn't have to follow the rules for summoning spells. A mutant gibbering mouther that can spew weird fluids that turn into grey oozes fit here, as do chaos beasts that transform the elemental spirits held within golems. The latter is not pretty as the golem and its warped spirit fight until one wins. If the spirit wins, it warps the golem's body and rampages with it. If the golem's magic wins, it simply deactivates until the spirit can be replaced. Breath weapons that animate objects are also a variant of summoning.

Curses are the "everything else" catagory. It has three of its own subcatagories:

Gear. The breath weapon has a negative impact on the PC's equipment, more than just damaging or destroying it. A fire breath weapon could melt and fuse joints in metal armor (good luck getting out of that). Magical items may gain side effects or actual curses. Armor may levitate, causing the PC to float away. A sword may transform into a sprig of dragon nip. Or a helmet may turn into a gremlin that gets into the rest of the gear.

Body & Soul. The breath weapon transforms the character or character's soul. Alignment changes, mental domination, mutations and such. And it doesn't have to be huge changes either- hair could be turned into a swarm of ants or spiders or dead skin flakes turned into a swarm of very tiny dragons.

Landscape. The ground and walls of stone may be melted by dragon fire, causing the PCs to sink into molten rock. A green dragon's breath may cause trees to die and fall over onto the PCs. A black dragon's acid may form pits, even right under the PCs. White dragons' cold forms black ice and may even freeze air when the dragon is very old. And blue dragon lightning may empower loadstones, increasing their strength and forcing metal armored PCs to fall to the stone (this could be really bad if the stone is in the ceiling of the lair).