Sometimes it's fun to look back at old missions that never were. There are more of those than the missions that receive funding and are launched, but many of those were influenced by the ones that were funded that came before. A great fountain of mission ideas is the Alpbach Summer School, held annually in Austria. Every year, at least two teams publish papers defining a complete mission concept as part of their capstone experience at the school. One published in 2014 describes a mission designed to look at Venus' tectonic activity, and even though the concept is over 11 years old, the scientific questions it sought to answer are still outstanding today.