(I'm mostly bouncing off of Chad's Answer here)
I think we're more on the same page than you think. It's just a matter of how far you want to push it.
Yes, discussing and working out storyline plots is an intrinsic part of "world building" — but when questions veer from specific, answerable questions about building alternative worlds into the realm of soliciting crowd-sourced plot lines, that's where you'll run into trouble. Let me run through a series of examples:
- Q: Can you simply scale up ants into a race of "giant insects?" — ON TOPIC
Very little argument here. The mainstay of this site will be questions that tap into your knowledge of technology/physics/geography/history/sociology/ecology, etc, etc in a way that makes sure your fictional world is at least plausible.
- Q: What conditions have to exist in a world to explain giant insects? — EXCELLENT
Yes, this question relates to plot building, but it's also good Q&A. It's somewhat akin to your question about credible ways a shapeshifter could gain/lose body mass. That's a fantastic question; love it. It taps into the expertise of physics and physiology and other areas in a way that is eminently "answerable" (even if the answer is somewhat theoretical and speculative). No problem there.
But here's where we run into trouble:
- Q: What would the world be like if it were ruled by giant insects? — TOO BROAD
This is soliting ideas for a plot line. That's not good Q&A. We're getting into a place where anyone can propose a vague topic of discussion, and participants can advance the storyline by posting stuff others may or may not find interesting. Then voting goes to what? Most creative? Best effort? People's Favorite?
While plenty entertaining, that's not really what we do.
I would suggest building up a really strong base of canonical, useful content of answerable questions (I hate that phrase) before starting to solicit broad plot-line discussions and crowd-sourcing story ideas. If people are simply (in a sense) guessing what might might help the author specifically, more than likely it is simply too broad.
But I'm glad you brought this up
Let's be careful NOT to make the mear mention of "plot" or "storyline" the verboten buzzwords of this site. There is nothing inherently wrong with asking questions that involve "story telling" or "plot line" in their content. All too often I see these types of on- and off-topic discussions elicit a kneejerk reaction anytime someone dares phrase their question wrong. Here, that trigger point would be "How could my storyline account for {x}" — and that's not at all what I meant by Hypothetical situations and broad questions.
A brief note to answer the original question — The generic subject of "story telling and plot building" is really only on topic here when the question is predominantly about the subject of world building. But very generic questions about any author might ask about the subject of plot building are more the purview of our Writer's site. I talk more about why that is here: What would be on topic here?