One of the great things about RPGs is how versatile and flexible they are. I still use my 1e DMG for every edition and permutation of the game. OSR products might just say they are to be used with "Any Old-School Fantasy Game". Heck, Flailsnails allows you to take PCs from different systems and just use them all at the same table. Converting adventures from one system to another is old hat for many GMs. You might borrow a setting from another system, or pick and choose mechanics from one game to add to another. That's what this post is about, mashing-up systems.
There are a LOT of games that share a supernatural theme: the idea that behind the happy veneer of the world, dark forces are at work. Monsters, dark cults, conspiracies...you name it and there is a game for it. This allows GMs pick their favorite mechanics from one or multiple systems and use the setting and background information from another. Some games seem like they were born to work together.
Some systems, such as Kult, are revered for their settings, not so much their mechanics. Other systems such as Cryptworld have fantastic, easy-to-grasp rule sets, but haven't been on the market long enough for a lot of supplemental material to be released. A GM might also just want to mix it up setting-wise, say porting World of Darkness mechanics into a setting that isn't quite as dark, maybe utilizing Chill's lighter, Universal monster-inspired adventures. The point is that all of these games deal with hidden reality and the exploration of it, and as such they can be blended in a hundred satisfying ways.
Recommended games: Kult, Cryptworld, Supernatural, World of Darkness, Hunter the Vigil, Chill, Call of Cthulhu (Cthulhu Now)
Many games deal with specific historical time periods, only the history has been twisted a bit or re-imagined, often with a dark or magical twist. Again, this allows the GM to borrow freely from multiple systems without having to make enormous adjustments. Obviously, the 1800s are not the same as the 1600s, but with a perusal of different equipment lists and skills (more on this later), the seasoned GM shouldn't have too rough a time making it work. Want to play Lamentations of the Flame Princess in a colonial American setting? Be a Boot Hill gunslinger in Victorian England? Try a mash-up of these systems and their supplements.
Recommended games: Lamentations of the Flame Princess, Colonial Gothic, Masque of the Red Death (Ravenloft), Boot Hill, Call of Cthulhu (Gaslight)
THE ROSETTA STONE
If you notice, I mentioned Call of Cthulhu twice already. That's because it is the "Rosetta stone" of mash-ups. The game has setting material for every time period from the Dark Ages to the present, the mechanics are easily convertible, and many of the scenarios are amazing.
LOST IN SPACE
Looking for a truly bizarre world for your intrepid space explorers to encounter? Maybe your Travellers happen upon the massive, rudderless Warden. Or crash-land on Jorune or Tekumel. Fish-out-of-water scenarios work with well with these somewhat inscrutable settings and allow the players to learn the cultures alongside the PCs.
Recommended games: Traveller, Metamorphosis Alpha, Starships and Spacemen, Thousand Suns, Stars Without Number, Skyrealms of Jorune, Empire of the Petal Throne
Gygax will always be best known for D&D, but his games Dangerous Journeys and Lejendary Adventure offer a lot of cool ideas for GMs to borrow. Dangerous Journeys is one of the most complex RPGs I have ever encountered, and you can almost just open a page at random to find some table or idea to incorporate into your games. Mythus Magick offers new schools of magick or variations of the classics that can rejuvenate your game. Apotropaists, Gray Mages, Astrologists, Exorcists...the list goes on. Lejendary Adventure offers new races (Oaf anyone?) and fun bits and pieces (knacks and quirks) that can easily be added into most fantasy games. The Epic of Aerth, the setting book for Dangerous Journeys Mythus, offers the GM an alternate version of our own planet to let his or her players explore.
Recommended games: Dangerous Journeys, Lejendary Adventure, D&D (all editions)
You may be reading this and be thinking, "But Froth, a lot of these games have very little in common mechanically. Conversion would be a nightmare." Well, I think most GMs will tend to stick with one system and primarily utilize the setting material of another. That said, I can offer some general tips with further conversions. As I mentioned in passing earlier, comparing lists of equipment from two sources is a great place to start, especially with historical games. You might dial back the damage of some Red Death weaponry if you are going to port into Lamentations of the Flame Princess. By doing this you also get an idea of current technology, accessibility to weapons or supplies, etc. You might port some skills over to your system. Compare and contrast skill lists and see which ones might benefit you to add or omit from the mechanics. Some games, such as Boot Hill, don't really have skill systems per se, so you might just borrow an entire system from another game, approximating the PCs ability with the skill. Games with percentile based skill systems are nice to use, as you can quickly convert to a "roll under" d20 mechanic a la AD&Ds proficiencies.
Most trouble will likely come when its time to bring in critters and monsters from different systems. I would start by looking for the monster's analogue within the rules system you are using. Common monsters such as zombies and skeletons will be represented in many games. You might also just find a similar monster and tweak it a bit. Eyeball the power level of a creature and its capabilities and wing it. Often an enemy might just be another human being, so use the same statistics and creation method you would for the PCs. Mash-ups can unfortunately take a little work, but its worth it to come up with something that really inspires you and your players.
I would love to hear of any mash-ups you yourself have run or been a part of! Leave a post!