A Blanket Thrown Over the Gears...

Role-playing has two elements.  First, the mechanical and/or quantitative aspects expressed through numerical simulation and manipulation.  But also more qualitative fare, like who, what, where, and when in a living world.  Who wants to inhabit a place of numbers when they can actually do things in a world of cause and effect - and do so in true, fantasy novel fashion?

Simulation gets us there.  But only if we understand gameplay mechanics as machinery over which we throw a clever disguise, much like a colorful blanket of imagination.  Gaming, from the very beginning, exploited this model, and in the capable hands of clever and creative players, the possibilities are endless...   

OK, so let's turn OD&D into a historical game.  Easy.  Make all characters either fighters or thieves.  No magic or non-human races, just humans.  Religion and magic use are social constructs, and magicians, in particular, are either deluded or (rather more likely) conscious charlatans.  The details will otherwise correspond to whatever historical period is ultimately chosen.

And we can prod this forward in time, treating firearms as various bows for damage purposes and assigning ammunition as befits the weaponry in question.  This covers anything from the Victorian era to the early twentieth century to contemporary times, noting that computers and modern vehicles are peripheral elements requiring few, if any, additional rules or new mechanics.

See, all we're really doing here is changing the blanket!

Mechanics make things go, but
without a mask, it's a pretty dull affair...

This enables everything from gangsters to the world wars (and you can take your pick).  But we can also roll back our prohibitions on magic to get something truly Lovecraftian if we want it...

Those having intelligence or wisdom greater than 13 may attempt to read scrolls or spell books, but must save vs. insanity or suffer gradual (and progressive) mental injury culminating in a full-blown malady per the AD&D Dungeon Master's Guide (page 83).

All such magic is ritual unless sworn to the service of some supernatural entity, whereas they become functional clerics having access to whatever clerical and magic-user spells the DM judges suitable.  Magic items may or may not be available, but will likely be dangerous and require saving dice against insanity as per the above rules, being ancient and of inhuman origin.

Obviously, demons, the undead, and any number of other monsters; whether extra-dimensional or otherwise, can be used.  Perhaps mind flayers and githyanki re-purposed and renamed to reflect a more nihilistic bent.  And those lucky souls having access to the first printing of Deities & Demigods can involve Cthulhu and company along with the assorted demon lords.  The stats aren't hard to find.

We're not saying you shouldn't buy Call of Cthulhu (we highly encourage it).  But we ARE suggesting that it isn't necessary if you have the requisite ambition.  Blankets are fun to make...

Pay no attention to the man behind
the curtain.  The action is really all up front...

Of course, truly ambitious DMs can push the campaign ever farther ahead in time for a futuristic experience.  Non-humans become extraterrestrials, with clerics and magic users being re-imagined as psionic beings with access to suitable spell effects (this can be challenging, but fun).  Perhaps dwarves are a burrowing race hailing from a sun-blasted world and forced to dig underground...

Guns (again) are bows for damage purposes, armor is advanced polymer comparable to its medieval equivalent, and spacecraft are no different from ships and similar vessels in your standard D&D game, subject to whatever additional rules you wish to add!

In many respects, rules are simply a mechanical frame over which colorful skins can be thrown.  Arrows to bullets to blazing plasma thrown from an alien blaster rifle - it's just damage and range mechanically speaking.  But the campaign; the adventures so created are many and varied, and firing blasters at squid-like aliens in some abandoned space station is FAR different from lobbing arrows at the King's Guard in the Halls of Lord Thoth! 

Face it.  This versatility is what made OD&D so exciting in the beginning and goes a long way towards explaining why D&D (through its latest iteration) remains the most popular game today.  Our hobby attracts clever, creative people who'll jump at the chance to throw that blanket over the gears!  And with so many systems now available; each with their own mechanics ripe for the changing, the fun is likely to continue for as long as the gears whirl...