Theme: Space (off planet, stars, clusters, nebula's) showing the galaxy, i.e universe backdrop.
This was expanded in Diablo 2, and I believe it explained some more of the story about the magic and perhaps its origins even beyond just earthbound protagonist vs order that are.
This can even be just developed as another bridge to future UberBoss fight's! Expanding more Bound on Account perhaps? Maybe.
Blizzard dev's are some talented individual's, Star Craft show they can make some great alien futuristic graphical presentations. I've been impressed. Perhaps Diablo 4 will have off-planet Battles.
Limited Time boss quests, a new Villain. A Charmer or Super Shaman.
Buff Stations are There for the fight and are in battle with the Boss.
Quest's which lead to battle the new Super Shaman: Killing Bosses in a certain order to gain access to a "super boss"
Co-Op is Needed finally, and another Gold-Sink" as clique as that sounds.
Ex Proposal: Added New Area's with new themed super boss and area's of conquest.
Expand on the Fallen Angel: Tyreal and Others and expand.
On confident on the vision, and believe the team could make a knock-out expansion update, or all together fixed game.
The game is great, and i'm proud of the Diablo 3 development team since the start, and I wish perhaps I can be of assistance, i would be honored.
Blizzard and The whole Community!