Wow… so, I just read a bit of the archives of CAD (I haven't read regularly in months), and I have to say, right now, I'll make a pact:
If I ever feel I can't go any further with the Bricktown characters, I will not - I repeat, WILL NOT - simply kill off Greg or Jill. To extend that, don't expect to see the demise of any OTHER characters any time soon (Even Stu, who's in his late 50s).
If I feel that continuing on will just drag things out and there's nothing new to do, I will simply find a good place to end and leave it at that, or transition to others and leave the current cast to live out their lives in the Bricktown universe.
I've never shied away from having dramatic moments, or even melancholic, but as a whole the series is meant to be optimistic; as such, you won't see any out-of-the-blue killing off of main characters just because I've got eternal writer's-block.
Mind you, the characters and setting in Bricktown are not in a static state - the characters age at a basically-natural rate, the story is linear, and I fully plan to keep a hard continuity.
This means, of course, that if I'm able to continue on with Bricktown for years and years (which I honestly hope to do), then character mortality WILL factor into things once it becomes natural (which would be in about 40-some years, as the characters are all 22-27 right now, so don't worry about that for a while).