I'm not expecially fond of so called cross-over. I don't look for a Walkurenritt played on an electric guitar, it takes away some of its substance and I don't prefer a Metallica song played with violins, it takes away an important part of its heaviness. Heavy, both in substance and sound is one of my favorite genre transgressing moments in music making: the lamenting of Elektra in the beginning of the opera that bears her name. Elektra's screams with which she tries to resurrect her murdered father Agamemnon qualifies for the term Heavy Metal opera.
Karl Böhm, who gave us a wonder of a Ring des Nibelungen in the 60s, closed his career (and life) in 1981 with a movie project of Strauss' Elektra (the movie was still in production when he died). In collaboration with director Götz Friedrich he produced one of the most intense and memorable operas on film that I'm aware of.