"You're in a tavern."
Classic start, right? We've all been in that tavern. The one where NPCs go to find trap-layers and dragon-slayers. The Craigslist for quests lines. But that's the problem. Tabletop RPGs live and breathe on their endless potential for innovation and surprise - the possibility that anything might happen - yet too often we find ourselves mired in the same plots over and over. Sure, rescuing captives, slaying monsters and uncovering Cthulhu cults are the meat and potatoes of GM-written modules, but players also know the story beats right off the bat: hear rumors, examine mysterious deaths, find monster and/or cult, kill monster and/or cult, then loot like there's no tomorrow. You need something new to keep your party interested - or maybe not. Maybe, like me, you instead turn to the historical record and repurpose something old. Because using a history book as your GM's guide can help shape your narrative into something players will remember for long after the last die rolls to a stop.
Module-Writing Made Easy
History is exactly that, a story. Every past event, from titanic clashes like the Napoleonic Wars to more personal events like the trial of Galileo already has a beginning, middle and end that clever GMs can easily adapt into an adventure. But better still, since these events actually occurred their relationships between cause and effect have a plausibility that GM-written adventures can sometimes lack. Plot twists come off as authentic rather than forced. Story developments have an innate sense of internal logic. Whether the players are key participants in the affair or merely caught up in the whirlwind of larger events, stealing plot details or settings from history can improve the depth of both your plot and general world-building. That's why the greats do it - George R.R. Martin is upfront about the fact that A Song of Ice and Fire is The War of the Roses with a fantasy paint job, and Ken Levine wouldn't have written BioShock: Infinite had he not watched the PBS documentary America 1900.
Just like these two, you can co-opt as little or as much history as you like - from full events complete with real people and places, to just the setting or even a single character, re-skinned to fit your world. Want to change history or bend the truth to make it more exciting? Go nuts. It's your adventure.
Steer Clear of the Usual Suspects
World War II is overplayed as a setting. So is Victorian London. Part of the fun of historical settings is playing in a world that's a little unfamiliar and where the players face new dramatic questions, and pop culture has already explored these events to a point where players will wind up reenacting or creating an homage rather than exploring new territory. You're here to play your own game, not GURPS: Band of Brothers. Besides, if the party is too knowledgeable about events or people from the period, it steals some of the tension from the story. The solution is to stretch yourself - try out another decade, or even another century. If your last Call of Cthulhu campaign was set during the 1920s, lay out the next one during the Red Scare of the 1950s. Rather than Victorian London, consider the Regency or the English Civil War. Further, even if your players insist on a WWII or Victorian setting, you can deliver a twist with a change in geography - send them to British India on the eve of the 1857 rebellion or Singapore under Japanese occupation. Especially look for periods of social upheaval or chaos, since it allows the party to go a little crazy without drawing too much attention to themselves.
Use Setting to Recast the Adventure
Changing scenes can take a fairly humdrum plotline and make it something memorable. Imagine you're writing a Hunter: The Vigil campaign where the players locate and disrupt a cult of Mages - pretty standard stuff. Now imagine a campaign where the hunters need to locate and disrupt the cult at Woodstock. They've got three days before the coven opens a portal to something horrible. Three days to find the Mages hidden somewhere amongst half a million people, all of whom are wearing weird clothes, chanting in made-up languages, and high on LSD. Think of the confusion and chaos that would occur. Consider the awesome soundtrack you could play in the background - chase scenes set to "Piece of My Heart," backstage knife fights during "White Rabbit" and a breathless showdown as Hendrix belts "Voodoo Child (Slight Return)." Use novelty to your advantage. Murder investigations seem fresh and new if they're in Ancient Egypt. Assassinating an enemy general takes on a different flavor during China's Warring States Period. And there's nothing to say Spanish Conquistadores never ran across Lovecraftrian horrors while trying to find cities of gold in South America.
Stir a Little History into Your Fantasy
You might think that this tactic doesn't apply to settings like D&D, but actually that's where this approach thrives. Dropping reality-inspired events into Greyhawk or Forgotten Realms works especially well because it both obscures the elements you're using and lends a new perspective to in-game events. Orcs are more compelling and sympathetic characters if they're sweeping in as part of a mass-migration like the Mongols or Saxons rather than just raiding a village because they're Orcs. Catching your players up in historical analogues also makes them feel like they're experiencing something momentous, with consequences that will continue well into the future. Cataclysmic change is also a great way to combat the complacency that sometimes sets in during a long campaign. If your players are getting too comfortable, knock them off-balance. Hit them with a natural disaster like the 1906 San Francisco Earthquake, forcing them to abandon their quest so they can rescue survivors and fight off armed looters. Have them find ore in an uninhabited mountain and spark a Gold Rush. During college, I GM'd an entire year of the swashbuckling fantasy game 7th Sea exclusively off of adapted historical plotlines, ranging from French court intrigue to the British bombardment of Copenhagen. As a finale, I threw the players into the Pazzi conspiracy, where they found themselves defending a stand-in of Lorenzo de' Medici from assassination, then hunted his assailants through the streets aside angry mobs. Array the entire might of history against your players, and you'll give them the satisfaction that they've changed the world - or that they merely survived the storm.