I had a similar thought about WoW. I'd love to tear through Azeroth using WoW's classes with an ARPG-level of power. Up the monster density, run have a roughly 75 or 100 hour campaign that runs across the world, and add some actual world-changing events (rather than having everything reset, or nothing change at all, because it's got to be there for the next guy).
I think it could work, although it would take more changes than I've glossed over here.
Mostly I think I just want to see each talent spec given a life and freedom in an ARPG. There really is some great variety and flavor between classes and even between talents specs within a class... or at least that was true at the tail end of WotLK before the Cataclysm talent changes. It's just that it all takes second fiddle to the realities of playing in an MMO: namely, the need to pull your weight and be effective. Some of that is lost as Blizzard constantly tweaks class mechanics, but most of it is crushed by players themselves, because There Can Be Only One Spec. (And Blizzard is complicit and even feeds that because they see what players are capable of, so they tune content for it, so players are forced into the most efficient specs for their class, and occasionally the most efficient class for their role.)