Carl Nielsen wrote some fantastic symphonies. He demonstrated progressive tonality around the same time as Mahler, but his music wasn't known outside Denmark for some time. I read he's on some piece of Danish currency. He outlived Mahler and thus developed influence from the modern innovations hapening at the time. I like his 3rd best, but his 5th is interesting historically as it calls for an improvisatory snare drum foreshadowing aleatory procedures.