Gesamtkunstwerk, literally “a total work of art,” was the concept behind Wagner’s operas as described in his essay The Artwork of the Future. He sought to unite and perfect the arts as he imagined the ancient Greeks had done. Poetry, drama, music, and visual art were to be combined in such a way that none dominated and each served the whole.
To this end Wagner actually constructed his own theatre so he could control the audience experience. We owe to him the convention of dimming the lights in a theatre so that our attention may be focussed on the performance and not on the latest fashions.
It appears that no one has attempted Gesamtkunstwerk in live performance since Wagner, at least not to the same scale, but I think movies may be its modern descendant. Though they lack the energy and immediacy of live performance, there is no question that films now combine the dramatic, visual, and musical arts to great effect. I wonder, if Wagner were alive today, which medium would he choose? Some of his stage directions, such as Valkyries riding horses through the sky, are still impossible today, but with CGI, who knows! Peter Jackson, are you listening?