Introducing Bobo the Mouth Breather!
Purpose: Destroy the world, by engulfing it in flames. Technically, I guess, it won't kill everything in the world at first level, but hey. 99% is close enough for government work. You'll have to wait until 4th level to be able to kill everything that isn't able to resist your chosen element of destruction. But those are the people you want hanging around in your post-apocalyptic dream world, right?
Here's a dramatic illustration of what it looks like:
Though in our case, we'll just be using a human instead of a colossal Bahamut summons.
The build:
Bobo the Mouth Breather
Level 1: Human Dragonfire Adept (from Dragon Magic) with Enlarge Breath (Draconomicon p70) and Clinging Breath (also from Draconomicon). We'll give him an 18 Con, and pick a breath weapon with a cone shape. No magic items or anything else is important. We'll give him minimum stats in everything else, just because it's funny.
Level 4 (alternate build): Human Dragon Shaman (from PHB2) with Enlarge Breath, Clinging Breath, and Lingering Breath.
Alternatively, we can play as any other class or race that gets breath weapons.
At higher levels, we'll take (in order): Heighten Breath, Shape Breath, Spreading Breath, Maximize Breath
The Nova Round (literally):
We breathe. Our breath weapon covers the world, and the world dies. End of story.
Explanation:
Enlarge Breath is a metabreath feat that explicitly allows itself to stack any number of times. Each time you do it, you get to increase the range of your power by 50% in exchange for waiting another round for your breath weapon to recover. For something the size of the Earth, it will need 48 million stacks of Enlarge Breath (yes, I calculated it), and then we will have to wait 5 years to be able to do it again.
At level 1, this will deal roughly 1d6(+1d3 from Clinging Breath) fire damage to all things on the surface of the earth. People with full cover naturally won't be affected, but this will set fire to all buildings, kill most low level creatures, destroy all the farms in the world, and generally be a world-ending event. All Caused by a level 1 adventurer.
...who will probably gain significant amounts of XP from killing all that stuff, and so will be better prepared for the retribution headed his way. Oh, and he's immune to his own breath weapon, so that's not a worry, either. Though he should probably pack some lunch.
At level 4, we gain lingering breath, which allows us to leave behind a cloud that deals half the damage of the original attack (2d6 now) in the same area. The cost is two rounds of cooldown per one round of duration added. So if we had 5 more years of duration, we gain 10 years of cooldown. Not bad - the world becomes a burning firescape for 5 years, with all creatures on the surface taking 1d6 damage. Eventually it will burn though most enclosed spaces, leading to a more total devestation. The only survivors on the surface will be people with evasion (and no need to sleep) and/or those with at least 6 points of fire resistance.
Also, since we'd be a dragon shaman at this point, we can choose acid or cold as our breath weapon. (As the poet said: Will the world end in fire or in ice? You decide!)
At level 6, we pick up Heighten Breath, which allows us to bump up the DC of the breath weapon arbitrarily high in exchange for +1 rounds of cooldown. So we can set it to DC500 or whatever, and thus kill off all those pesky rogues and monks as well. Our weapon now deals 3d6 damage, too.
And so it goes... eventually we'll want to get the ability to turn our breath weapon into a spread, and maximize the damage, and get better breath weapons, but you all get the idea. By 20th level, he can throw out a cloud of acid that deals 200 points of damage to every square on and inside the planet for an arbitrarily long period of time, meaning that eventually the whole thing will become disintegrated. While also encasing the world with hurricane-force winds (via Tempest Breath) Nobody can do anything about it - it's not a spell, so it can't be dispelled, only resisted.
So at the end of the day, Bobo the Mouth Breather gets to decide what kind of planet he wants to live on, and gets to reshape it to his whim. He could even leave safe areas for his devoted followers. Not bad for a naked human with an 8 intelligence, eh?
This guy gets to be king of the apocalypse?
Conclusion: Don't complain that a DM would never allow this. I fully agree. This is purely for entertainment purposes only.
I'm going to try this for my game tomorrow just to mess with everyone.
The section on metabreath feats says:
To take a metabreath feat, a creature must have a breath weapon whose time between beaths is expressed in round.
The enlarge breath feat doesn't say it stacks with itself, and the section on metabreath feats says:
If a metabreath feat stacks with itself, this fact will be noted in the Special section of the feat description.
I don't see anything in your build that eliminates the need for line of effect. This means the horizon will protect at least half the world.
Fire deals half damage to objects, and hardness is applied after it's halved. This means any object with hardness 3 or better in unscathed, unless it has a special vulnerability to fire (as wood does). Any stone structure will be fine, and will block your line of effect.
The entirety of the underdark will be fine.
The surface of water block line of effect for fire, so you won't be getting any aquatic life forms.