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Hero
Great topic Ari Thanks! Have you answered your own question yet?
Heh. I have not, because I'm having trouble whittling down my list to a couple of favorites. I come up with campaign concepts the way many people come up with characters.
Some of them are just brief ideas. Like the idea of a "vs. kaiju" campaign where the PCs are totally overmatched by the newly appearing giant monsters and must go locate the tarrasque and lure it back to take on the invaders in a "Godzilla vs. Mothra" scenario. Or a campaign where the PCs have all died and gone to Hell, and Mephistopheles brings them back to hunt down people who have avoided or escaped Hell. D&D meets Ghost Rider + Brimstone.
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Then there's this:
The PCs (who would start somewhere in the 4th-6th level range, and thus already have a rep) are approached by dwarves. The dwarves are seeking to regain a homeland, very much an Erebor or Gauntlgrym thing. The city was lost generations ago, and is in the hands of gods-know-what. It is also cursed; the dwarves themselves cannot return until the curse is broke, which is why they're gathering outsiders for this. The PCs, and other humans who assist, will be granted lands and titles within the dwarven kingdom.
Some (but not all) objectives and plot arcs would include...
1) Clear out the upper levels of whatever has claimed them, so the PCs can use it as a base and start moving in workmen, builders, etc.
2) Research the source of the curse and ways to break it.
3) Recover lost dwarven relics throughout the region so they can be returned to the city, where they have religious or magical significance.
4) Gather other treasures to fund the rebuilding and jumpstart the economy, as well as necessities for farming, ranching, crafting, etc.
5) Gather other dwarves to come live here once the curse is lifted.
6) Eliminate various threats to the nascent city nearby, like monsters, orc tribes, etc.
7) Deal with political machinations among the factions of the new mostly dwarven government.
8) Start building up their own properties; maybe the cleric PC wants to have a temple here, the wizard a school, etc. These must be constructed and incorporated into the growing society.
9) Eventually reclaim the deeper levels of whatever holds them, and clear the surrounding Underdark of threats.
10) Open diplomatic ties with neighboring lands, above and below ground.
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Or this:
D&D Canterbury Tales. A campaign in which the PCs are part of a long journey or pilgrimage. Adventures alternate between events occurring to/on the pilgrimage, and the stories the PCs are telling each other to pass the time--stories in which the PCs are effectively "stand ins" for the characters being talked about, and still level/develop accordingly, but can thus find themselves in all manner of different environments/situations that might otherwise not be possible in an internally consistent setting.
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And I have others, as well.