I’m not quite sure what you mean by “speculative fiction,” but here are some really cool books that involve various musings on what might be or what might have been:
Invisible Cities, by Italo Calvino
Einstein’s Dreams, by Alan Lightman
The Aleph and Other Stores, by Jorge Luis Borges. This one’s especially recommended if you have a fondness for math. Well, really, any of Borges’s fiction would fit in this category. In fact, you might really enjoy *A History of Infamy[i/], though that’s not fiction, really.
The Name of the Rose, by Umberto Eco (This one counts as a mystery, too.)
Anything by Julio Cortazar.
Fiction that combines history and/or memoirs with fantasy and/or speculation:
The Woman Warrior: Memoirs of a Girlhood Among Ghosts, by Maxine Hong Kingston
The Posthumous Memoirs of Bras Cubas, by Machado de Assis. I haven’t read any of his other works, but from what I’ve heard, almost any of what he’s written could fit into this category.
Such a Long Journey, by Rohinton Mistry
Any of the novels by Clarice Lispector
The Tin Drum and Dog Years, by Gunter Grass. If the books seem bogged down in history for a bit, have patience–understanding the context laid out in the beginning pays off later.
Lots of stuff by Naguib Mahfouz. Check out Palace Walk, the first of a series (the second book in the series isn’t bad, either, but the last book is depressing and a bit polemic for my tastes. I’d skip it, if I were you.) Mahfouz’s Children of the Alley is interesting and hard to characterize (it’s a semi-allegorical, semi-fantastic reframing of the social histories told in the Old Testament, the New Testament, and the Koran. There’s also an allegorical retelling of more recent history. I’ll leave it at that.)
Men in the Sun by Ghassan Kanafani. This is a very well written and moving collection of short works–several short stories and a novella–about Palestinian exile and loss. You can read more on Amazon.com. The reader reviews say more than I’m prepared to write up here and now.
And, if you like Ray Bradbury, you probably enjoy good writing that focuses on the psychologically weird and/or fantastic. You might like writing by E. T. Hoffman, Carlos Fuentes, and Jose Donoso.