Just from the blurb in the help center about what's on-topic, changing a question to be about 'historiography' should make it on-topic:
History Stack Exchange is for historians and history buffs. If you have a question about:
Then you are in the right place.
However, at its core it seems like you're trying to draw a line between questions like "what could have happened?", and questions like "what did X believe could have happened?" or "did anyone think X could have happened?".
"What could have happened" questions just lead to broad, off-topic answers: either we give our own speculation as random StackExchange users, or we cite sources making the same sort of speculation we could do. People could say yes or no based on any number of hypotheticals, all of which could be completely unrelated to what the relevant historical figures knew at the time or what later examinations thought was possible.
However, "what did X believe" questions have only a few on-topic options: either X thought about what could have happened, X didn't think about what could have happened, or we don't know what X thought. Similarly for "did anyone think about X": either some historical figure thought about X, or there were reasons for thinking X wasn't possible. For these types of questions, it doesn't matter what potentials existed, what historians knew in hindsight, or what we StackExchange users can think of: answers will either cite historical figures' thoughts about some hypothetical, or will try their best to show why the the hypotheticals weren't considered.
So, to use a recent question about an alternative outcome to the WWII Pacific campaign as an example: Had anyone envisioned a "wipeout" scenario for Japan in the Pacific for World War II?
- If a question explicitly asks for hypotheticals made by contemporary sources (military leaders of the relevant countries, civilian thinkers/reporters/etc.), then it should be on-topic
- If a question asks about post-conflict examinations (later historians, military leaders shortly after the conflict, etc.), then it should be on-topic
- If a question asks for hypotheticals that could not have been known at the time (such as future technology advancements/discoveries) or for examinations that that rely on events that didn't happen (if so-and-so succeeded at X, what would they have thought about Y), then it's probably off-topic