Franz Joseph claimed the title of "Emperor of Austria" from 1804 to 1835. The only "event" of 1808 that I've found was his marriage, on January 6, 1808, to another first cousin, Maria Ludovika of Austria-Este (December 14, 1787 – April 7, 1816) with no issue. She was the daughter of Archduke Ferdinand of Austria-Este and Maria Beatrice d'Este, Princess of Modena. (see Wikipedia)

Commissioning of "grand projects" was sometimes a rite of celebration of such marriages, so it is possible that the Staatsoper was ordered to be built by Emporer Franz Joseph in 1808, although most sources say that the actual building of it took place between 1861 and 1869.

For a building of this magnitude, that's not an uncommon delay between placing the order and getting the bricks.

The architect was one Edouard van der Null, but I don't find much of anything about him except that he died in 1869, the year that the original opera house was reportedly completed.

John