The degradation of any form of Art seems to peak at the same point at which the general subject matter becomes most mundane. I don't know what the cause-effect relationship here is, but the two certainly go hand in hand. Certainly, in my own music, I've been working up the thematic content/actual purpose of the creations from "describe Winter/Death/Gothic themes, reference Romanticism" to "communicate absolute necessity of the eternal", and I believe it can go further still. My suspicions ever point towards the incumbent and increasing necessity to simply ignore the overground/mainstream, and remove any and all desires for any amount of fame (though notoriety may not go amiss).
Generally, in terms of "exploration of style", Extreme Metal (specifically, the Extremity of it) seems to be rather tapped. I like the idea of "Metal Symphonies", and would say that many of the best Metal albums are, in effect, just that, though without the rigidity of form of many Classical Symphonies. The focus must be on the effect and quality of the music, rather than on "how extreme it is". We have all of the extremity we could need, already mapped out for us, thus I would say it is time to focus on (compositional) complexity, with a view to using music as a method of describing or explaining fundamental ideas and ideals.