Frederick the Great was planning to marry Maria Theresa of Austria in the 1730s but he was persuaded to marry another woman. What if Theresa and Frederick would have married?
A Prussian-Austrian empire would have been a major power. How would this have affected the Polish Partition, eventual revolutions, Napoleon, German unification and so on?
Several wars would have been avoided or would have turned out different (such as the War of Austrian Succession or the Silesian Wars), and therefore I think it's fair to say that a marriage would have caused a major change in world politics.
What do you say? How long would such an empire have lasted, what problems would they have faced, how would the other great powers have reacted?