I was recently asked whether or not it was a good idea to combine stories - in other words, to take an existing story and in some way join it with another one, rather than let it stand alone as a work in its own right. There are examples of this in the world of literature - the one that immediately comes to mind is Tolkien’s children’s tale The Hobbit, which quickly became linked with, and then very much a part of, the complete world of other stories he had been working on for decades prior to its publication. It’s a fascinating subject and one which I probably can't do full justice to without further study, but here are some initial thoughts.
I always think (and I go into this much more in my book How Stories Really Work) that our thinking is slightly askew when it comes to fiction. We tend to think of stories as almost mystical things (which on one level, of course, they are) and often fail to see them as practical communications (which on another level they also are). When it comes to the question of whether a story should be blended with another or left to stand on its own, my first instinct is to ask "What are you trying to DO with the story? What effect is aimed at here?'
If a story has developed around a core message or core vacuum (desire, gap, hole, loss, threat etc), then all its pieces should be aligned with that core and should serve it. If they do this powerfully and effectively, then I would say leave it alone.
If a story, on the other hand, would have its core strengthened by being combined in some way with another tale - perhaps as part of the same sub-created universe, or featuring the same characters - then I would say combine it.
In both cases, the story isn't the thing, the core vacuum or message is the thing. Be guided by the meaningful impact rather than the mechanics of the tale.
One factor that crops up with the use of the same characters, which we see in ongoing serials or series all the time, is 'character fatigue', which arises because a character (as you know) isn't what many writers and readers suppose it to be, but is rather a construction of vacuums. In a well-told tale, a character has his or her vacuums (desires, needs) fulfilled in some way (or intentionally left unfulfilled) which brings the 'life' of that character effectively to a close. Characters are clever illusions anyway, and once their trick is performed, they are done. To stretch a fragile construction of vacuums further and further, into more and more adventures, sooner or later produces a wraith-like figure, a fabrication whose lifetime should have ended long ago. The tired old tropes of television dramas, and the ridiculous re-booting taking place in comics these days, are both symptomatic of this. Conan Doyle himself grew so sick of Sherlock Holmes that he had him killed off, only to have to bring him back due to public demand - but the later Holmes, like the later version of most over-extended fictional protagonists, is more of a caricature, a two-dimensional figure, not the powerful, vacuum-driven creation he was in the beginning.
Let me know what you think!