Choosing between a campaign setting or a new game?

What are few things you consider when deciding whether to develop a new campaign setting for an existing game or going full out and making a new game?

I've only made a full out game once. As a child in the early 80s, we had played Top Secret a couple of times but I wanted to play a more GI Joe kind of game, so I made one. I statted out various vehicles and me and a friend played it once. Even so, I don't consider the exercise a failure as it was an interesting diversion.

These days, I never use existing campaign settings as I prefer creating what I want to researching the whole backstory of someone else's creation. I rarely give much thought to the ruleset, as I have so many great systems at my disposal that I feel my time is better spent choosing the ruleset that evokes the mood and theme that I want.

So in answer, if I felt that the tone and the pace of the game didn't match anything that I had, and I really wanted to share that with my players, then I suppose that I would think about custom designing a game to fit.

It takes a lot of time and effort to start a new world, and a lot more to design a new game (and I've tried lots of methods to minimize that, from designing as you need to, all the way to sharing the responsibility with players in a variety of ways). In the end, I start RPGs because of game systems I find that I like or want to try out, and in those cases I always take premade worlds to go with them because, again, its time intense to make anything myself. Of course, when the game starts the world becomes mutable - the way I run premade worlds is so that, once the first dice is cast the game world's history and future and shape becomes our own (we use premade settings as springboards).

I've designed my own system once (you can find the current rule book on these forums). I started from Wushu, a simple system I like, and added some more 'gamey' aspects to it (because I wanted a story telling oriented system that had some embedded strategy). I mostly stopped working on it because the two games I was running stopped happening and because, again, it's time intensive to design a game system!

I think it depends on the setting. Playing Warhammer Fantasy RP 2.0 I would smile, oh, ah, grimace, and snare at the name drops to Warhammer Fantasy lore mentioned. The rest of the group didn't really play the miniature war gaming version or know about it so sometimes things were lost on them, other things seemed amazing due to that though. On the other hand a custom setting is something created just for your party which is also cool. There might be other characters like yours but no one else has played in your world. My example of this was a space opera game where I played a con artist disguised as a project manager, it's easy to steal information when you request reports on special projects. And if people question you use buzz words and state you'll schedule a meeting with them to discuss the framework of what they'd like to discuss. Mention you'll need to find someone to take minutes of the meeting first and most people in real life will shy away from RPing being in a four hour meeting... (Did I mention my PC looked like the boss from Office Space?). So yeah I think it depends on the group and how much time you have.