My first thought on the notion of 'Speculative History' was to literally to supplant speculative motivations behind a real life event, rather than a 'what if this happened instead of that?' stance. I guess that would be my suggestion to anyone that might be struggling to come up with an idea: what event in history has fascinated you and then come up with a leftfield version why it actually happened.I agree it's very difficult to parse that out. As I reflect on it, I'm not sure there is an easy way to change history and not have it some kind of alternative history. The best I can come up with is to work with the normal timeline of a story, say the 1850's, but change one thing in it, say, the electric guitar was invented. How would that change things for classical music, or some such as that.
i.e. Titanic only hit the iceberg because the White Star staff onboard had actually run out of ice for their whiskies, so demanded the ship alter course to intercept one!!!
Anytime the word 'speculative' appears in a challenge, my first instinct is always God conversing with someone famous, for some reason.