I was writing a reply to another post and realized that my thoughts on this subject may be better off as a new discussion.
We encounter a lot of Intellectual Properties that seem like they'd make a great game, but they end up floundering. And why wouldn't we look at our favorite books, movies, and shows and not want to play in them? It's what we do.
As being discussed in the Carnival Row thread: theme is important.
Your setting is the IP itself (Carnival Row, Harry Potter, Buffy, Battlestar), but the theme is the question that needs answering.
How do we defeat these cylons?
What was life like as a student during the rise of Voldemort?
The problem with some IPs is that the theme question becomes fuzzy. Take Dresden Files for example. I fucking love this series. A coworker and I chat about it daily (I'm taking my time on the most recent book; he's about 7 books in). But I've had to admit to myself: it doesn't make a good game.
The reason is that when you take away the set dressing, it's just another urban supernatural game. You might as well be playing 'WoD with limitations.'
And while I've, for a long time, felt that's because some things lend themselves better to plucking a metaplot out of the air (Star Wars: pick an era, you have an antagonistic force baked right in), I think it's actually a much easier answer:
Is the entire story contingent on a single hero? Or is it about a group of people?
Buffy had a cast we all loved, but when it came down to it: it was about Buffy. Take her out and the whole thing falls apart. She was the star the rest of the solar system revolved around.
But take the Magicians, for example: their world lends to conflict. The last season of the show is a wonderful example. We saw groups outside of the main cast affected by the magicians vs hedge witches and magic shortage. Affected by and working to resolve, both.
The story exists without the star. This is the case with something like, say, Star Trek. Take away whomsoever you might think is the star and the story continues.
Now, that doesn't mean the others are bad stories, but it does mean that (IMO) they make the challenge of building a game that much more difficult.
You need to have that theme question. And when I consider the fact that I can go on Generic WoD City Game #17, a Dresden Files game, or a Buffy game, and make the exact same character on all three ... the theme falls apart.
It's why I absolutely believe that metaplot has to come with the game. It doesn't have to be a metaplot you roll out as massive, world-altering chapters (like Arx does), but it has to be present because the characters (and players) need something that keeps them within the world and makes them feel relevant.
But for me, the very first question when approaching an IP: is this setting suitable for an ensemble cast? In a way that enables each and every person to have an impact as opposed to being simply an observer?