I agree with the idea, but it's a functionality easier to implement via glyphs. Hey, remember those, cool bits of customization Blizzard pretty much threw away?
Instead of having, for instance, a whole Necrolord skin for a Warlock, via some content you could be able to unlock Glyphs that help you achieve that effect. Glyphs that replace Imps with Skeletal Archers, Hand of Gul'Dan with Corpse Explosion, Demonic Tyrant with Lich. Windwalker? Change Rising Sun Kick into Haymaker, Fists of Fury into Mad Flailing, Earth/Wind/Fire into Call Backup. There we go, some glyphs and you've got yourself a Brawler instead of a Monk.
Also, something to do for those sad Scribes, damn, profession created for Glyph could use some Glyphs to make.