A mash-up roleplaying game (RPG) campaign is one of those things that can have any number of positive or negative connotations attached to it. The basic idea is straightforward enough; you take two different genres, and combine them into one RPG campaign world. Some published examples of this would be Deadlands (Western plus horror), Nights’ Black Agents (espionage plus vampires), Shadowrun (fantasy plus cyberpunk), and we’ll stop before this post simply becomes a list of games that are A Meets B, At High Speed.
The advantage of doing a mash-up is obvious: it allows for a different take on classic tropes and genres. What will your player do when the outlaw gets up again after being shot by the lawman? How will you run a black-bag job against a Dracula that knows all the tricks? Does an Orc prefer a shotgun, or a grenade launcher? And the primary disadvantage is just as obvious: a hybridization like this can have some fairly obvious seams — or fault lines. If, say, your homebrewed Western/pulp campaign isn’t properly thought through first it might disintegrate the moment somebody tries to run with a trope that works in one genre, but not the other. I mean, imagine how your player would feel if he was the one in that example that was carrying the sword.
So, as always, let’s try for some practical advice:
First: there’s no need to think too hard about some of this stuff. Part of the fun of a mash-up campaign is that it increases the number of cool things that a party of adventurers can do, and can even give them new kinds of cool things to do. In exchange for this, the PCs can avoid kicking the tires too powerfully, yes? …And this means that the GM should feel free to preemptively nix the infamous Well, if we were able to do X, then surely we could do Y, which is why Z shouldn’t have happened and I can do a million points of damage to the bad guy now argument so beloved in so many RPG campaigns. A slightly illogical campaign — especially one that’s proud of being slightly illogical — is not always required to follow the prosaic, mundane rules of logic. Even if the players try to wheedle otherwise.
Of course, A mash-up campaign only looks weird from the outside. It may very well be weird generally; but both the player characters (PCs) and non-player characters (NPCs) are likely to be simply more accepting of the peculiar rules of their universe. If you have a game where magical creatures are openly operating in a noir setting, unless this just happened then people are going to have largely gotten over the first shock, and have now come to terms with it. What this means in term of gameplay is that the gamemaster (GM) should not feel shy about mildly reminding the PCs that there’s no need for their characters to wander around in a constant state of mild confusion and disbelief (which can easily lead to the players deliberately trying to poke holes through the whole campaign concept).
Relatedly: never underestimate the power of a sleazy rationalization. There’s undoubtedly a reason why a potentially campaign-ending cross-genre contradiction can’t actually occur in your mash-up campaign: you just don’t know what it is, yet. When in doubt, ask your players why there isn’t one. They will almost certainly like their answer than yours anyway, and if all you’re interested in is papering over the cracks until the campaign is over, sometimes that’s all you need.
That is the last point, really. And this, too, shall pass. The shorter the campaign, the more esoteric you can make your mash-ups. Want to run a game about psionic mice that create giant steampunk mecha to fight the Great Old Ones? You can do that without a hitch, if you accept that there’s going to be a hard expiration date on that campaign before it collapses into a pile of vaguely squeaky gears. When you start to feel the campaign shudder under your feet because the internal contradictions are starting to pile up beyond your ability to finesse those contradictions, go ahead and run the endgame. Make it memorable, loud, have the PCs get an appropriate ending… and then go play something else.
Because, hey, it’s a game. Play one until it’s no longer fun, then go start a different one. Maybe make the mice biker werewolves looking for the Treasure of the Sierra Madre. Could be entertaining.