A philosophical question.

We're all familiar with fictional places in the mostly real world - think Gotham City, or Syldavia (comic books seem to be the most prolific users). I'm interested in developing such places. It's the context that helps make them interesting.

Problem is, I can't go too far in my development because I run into blocks dealing with fitting these places into real geography (and history). Most such places are spatially vague, made for storytelling, not mapping. They often rely on the reader's assumed lack of geographical knowledge.

The problem comes when one tries to think how these places would affect the real ones. There's a conceptual mismatch here: trying to alter geography while keeping as much other geography and history intact as possible. These aren't supposed to be about alternate history; that destroys the point for me.

I realized a long time ago that all fiction is alternate history, whether or not it declares itself to be. This becomes a problem for me when I try to make other types of fiction.

So how do you do it? Make fictional places that can be a part of a mostly-real world, that is.