This phenomenon is popular in serial fiction such as comic books.  Rival publishers DC and Marvel Comics regularly publish series called, respectively, Elseworlds and What If.  For example: What if Professor X and Magneto had formed the X-Men together?  What if Superman's space capsule had landed in Soviet Russia (Superman: Red Son) or in 18th century colonial America?

The "what if" question has spawned an entire genre of historical fiction, called alternate history.  The website Uchronia.net features an annotated list of 2700 novels, stories, and essays that explore the "what ifs" of history.  One of the most common themes is: What if the Nazis won World War II?  (In one novel, someone goes back in time to prevent the birth of Hitler, but someone else becomes the Fuehrer and succeeds in wiping out all the Jews of Europe.)  In alternate history, every juncture of history may be scrutinized.  What if England had suppressed the American Revolution?  (Presumably that would have been easier if Superman had landed in colonial America and been raised by Tories.)  Israeli history is examined as well.  What if in 1948 an Arab state had been founded next to Israel?  What if in 1967 Israel had annexed Judea, Samaria and Gaza, and granted citizenship to the Arab residents?  Interestingly, the spread of the Zionist dream a century ago was partly due to Old New Land, Theodor Herzl's utopian novel that dared to ask: What if there were a Jewish state?

The rabbis dabbled in alternate history as well.  Several statements in the Talmud and Midrash start with the words ilu (if only) or ilmale (if not for).  For example: Ezra would have been worthy of having the Torah given through him, had Moses not come first (Sanhedrin 21b).  Had Hezekiah sung [thanks to God] on defeating Sennacherib, he would have become the King Messiah (Shir HaShirim Rabba 4:3).  Had the Jews returned en masse from Babylonia to Israel, the Temple would not have been destroyed a second time (Ibid. 8:9:3).  If only the Jews would keep just one Shabbat properly, the Messiah would come (Yerushalmi Ta'anit 1:1).

An extreme example of rabbinic alternate history appears in the very first comment of Rashi on the Torah, citing a midrash (Tanchuma Buber, Genesis 11): "Rabbi Yitzchak said that the Torah should have started with the verse, 'This month shall mark for you the beginning of the months' (Exodus 12:1), since that is the first mitzvah commanded to the Jewish people."  In other words, theoretically the Torah should have left out Genesis entirely!  Rabbi Dr. Jonathan Sacks, Chief Rabbi of the United Kingdom, points this out in an essay called "Fundamentalism Reconsidered" (Jewish Action, Summer 1991, p. 24): "Evidently R. Yitzchak was prepared to contemplate a Torah which omitted entirely the narratives of creation, the flood, the patriarchs, the exile and the first stages of exodus."  What is the significance of this hypothetical revisionism?

One could suggest that here, and in general when the rabbis ask "What if," it is not primarily an intellectual exercise, but an educational one.  When we imagine what might have been if Hezekiah had thanked God, not only do we mourn a lost opportunity for the Messiah, but we also remind ourselves not to miss our own opportunities to thank God.  When we imagine a Torah without Genesis, we appreciate all the values that we learn from its stories.  When we imagine the Messiah arriving because of mass Shabbat observance, we remember that the power of Shabbat is not only restorative but redemptive as well.

It is not surprising that the Passover Haggadah, which is full of educational and experiential opportunities, includes alternate histories.  Near the beginning of the Maggid section, we proclaim, "Had God not taken us out of Egypt, then we, our children and grandchildren would still be slaves to Pharaoh in Egypt."  In other words: What if the Exodus had never happened?  This alternate history seems to say that we would not have been freed even much later, but would have remained slaves for thousands of years!  (Interestingly, famous sci-fi author Robert Silverberg wrote a short story, "To the Promised Land," which posits exactly that.)  Though some of the commentaries reinterpret the statement, the alternate history is certainly meant to be thought-provoking.

Later, at the end of Maggid, we sing a song filled with alternate histories.  We imagine: What if God had took us out of Egypt, but not punished the Egyptians?  Dayenu -- that would have been enough.  What if God had punished the Egyptians, but not attacked their gods?  Dayenu.  We proceed to describe over a dozen alternate histories of the Exodus, and follow each one by proclaiming Dayenu.  (This means either that each situation would have been enough for us to need to thank God, or that each would have been enough based on what we deserved.)  Then we end with a flourish, listing all fifteen favors, and emphasizing how much we owe God for having done all fifteen of them for us.  Apparently, it is by imagining a string of inferior alternate histories -- what didn't happen -- that we can be grateful to God for the one true, wonderful story of the Exodus -- what did happen.  Once again, we see how the rabbis use alternate histories for spiritual purposes. 

What if?  That is a question which can elevate the soul.