Don't forget that the Temple district was once home of the Templar knight, the Marais was a marshland area. On the left bank, the oldest churches of Paris can be converted into pagan temples (là, je kiffe). And the chatelet as a fortress, le Louvre a castle or a ruin to be explored. Last but not least, les Halles that I personally call the Moria due to the height of the vaults and the constant presence of hordes of goblins customers.
Those are just some raw ideas, I'm bored in a bus between Paris and Nantes.