Lately it seems like a lot of the freshest genre works are mash-ups of well-worn ideas. Whether it's "police procedural with elves" or "superhero movie with Cold War intrigue." But why do some genre mashups rule, while others fail horribly?
Is there some magic secret to mixing and matching genres in a way that actually works, instead of just feeling like cliches piled on top of cliches? Does mashing up genres automatically make them fresh? Is there a right way or a wrong way to make genres collide?
What follows is some theorizing — as always, feel free to disagree in comments. I wasn't wearing my Infallibility Hat (TM) when I was jotting down this stuff. Although there was coffee involved, which is the next best thing to infallibility.
Anyway, in a nutshell: genre mashups don't even remotely rescue well worn genres from seeming cliched or tired. In fact, mashing up genres can only make them seem more cliched, if it feels like a lot of rote elements are being trotted out because it's expected of one of the genres you're sampling. And there's almost nothing worse than a genre mashup that feels like a pastiche cluster. Beware the pastiche cluster, for it will turn gooey. On the other hand, sometimes splicing some foreign DNA into a genre will help you get at the heart of what made the genre work.
Now for a longer answer...
You can't really answer this question, without delving into what genres are, and how they work. For writers and creators, genres are a toolkit that let them tell stories — but that's not what they look like to audiences. The audience doesn't watch a cop drama and think about what genre tools the creators are using — except to try and guess what's going to happen next, or to spot obvious cliches.
In a sense, an audience only notices genre as part of the fabric of the story if it's going wrong. If genre goes right, it's almost invisible to the audience, except when they have to find a way to describe the story afterwards.
And I'm going to climb out on a bit of a limb here and say that one of the ways you can tell a genre is starting to get played out is that people are too hyper-aware of it as a genre, while they're experiencing it. Genres that lose their invisibility are no longer genres that have storytelling power.
Of course, nowadays we have the internet, which has democratized criticism to some extent and made people more intensely aware of cliches and genre expectations. So it no longer takes as much time for yesterday's brilliant fresh story conceit to become today's "oh no, ballerina unicorns again."
If you think about it, story genres are like breeds of dog, except not really. We've bred stories more and more selectively, to maximize one particular quality. Some stories are prized for their extreme emotional generosity, others for their ability to hunt across long distances. Some types of story are especially cherished for how well they can point at stuff. And so on.
But the danger in breeding stories to emphasize one trait so heavily is that you wind up with an overspecialized breed, that has profound weaknesses in other areas and has a hard time reproducing itself. (At the risk of pushing a metaphor past the point where that dog won't hunt.)
So anyway, the goal of splicing together two or more genres is to get at what was cool about those genres, which over-familiarity may have obscured. And to make people excited about superheroes or Westerns or vampires or zombies again. But most of all, the goal is to make genres invisible again — because like I said earlier, when peopel are enjoying the story, they're not thinking about how it does or doesn't conform to any particular genre. (Not that it's not fun to consider such things, but it's not necessarily how most people enjoy genre entertainment.)
And maybe the first step to making genres invisible within a mix is to stop being self-conscious about them. Stop thinking of them as a laundry list of stuff that happens, or as a wreath of decorative bangles that you hang on your story. And start thinking of them instead as building blocks of storytelling that bring with them their own atmosphere or possibility for surprise and excitement. A genre is not just "a list of things that always happen," it's a mixture of world-building and character archetypes and the sort of ideas about the world that you need a host of stories to work out.
What hidden beauty of Genre A can you expose by bringing in Genre B? What tired aspect of Genre B does Genre A allow you to do away with? What do these genres have in common — and where can you find the ragged seams where they don't quite fit? (Those ragged seams may be where your story is. Focusing just on where the two genres mesh perfectly may result in a less exciting story, over all.)
Most of all, what is the story you want to tell, and how are these two toolkits going to help you tell it? The genre is not the story. The genre is just the basis for the story, or one piece of the story. Inception might be a heist movie at its core, but it doesn't slavishly stick to the "Ocean's >10" formula. The need to focus on telling a unique story, with its own clear and emotional arc, is even more urgent when you're handling two or more genres instead of just one — because two genres means two rubrics for you to be following slavishly, if you're not careful, instead of one.
Somebody had to be the first person to come up with a hard-boiled detective story, or an urban vampire story. That person had to come up with ideas from scratch, and probably didn't really think he or she was creating a genre. I'm always of the opinion that you're writing your best stuff when you come closest to that mindset, of blazing new trails and spitballing new ideas. And maybe out of all that genre mashery, something will emerge that we'll have to call by a whole new name. We can hope, anyway.