Can anyone think of any metal songs that adhere closely to this classical approach?
A partial definition from grovemusic.com :
A form founded on repetition...in which a discrete theme is repeated several or many times with various modifications. A theme for variations, rarely shorter than eight or longer than 32 bars, may be a melody, a bass line, a harmonic progression or a complex of such elements. Sets of variations may be freestanding, independent pieces, most often for solo keyboard but also for orchestra and chamber combinations, or they may be movements in a larger work...
Nothing obvious is coming to mind, or at least nothing that very strictly adheres to this idea. Burzum songs like Det Som En Gang Var and Tomhet seem to come close, but there are clear breaks from the original theme.