When I think of a storyline that seemed to have potential, but then fizzled out, I always think about an elf. In the 1970s, Steve Gerber was writing THE DEFENDERS, and there was an ongoing subplot about this elf who would show up at different locations and shoot and kill some random person. It may have been under a different writer (I don't remember that well, and I don't have the issue), but rather than letting this develop into some great enemy for the team, all of a sudden one issue, we see the elf get run over by a truck....and killed. And that was it for the elf storyline.
So I was wondering if anyone else has a contender for a story that had potential....but then fizzled out.