I agree that Diablo III should be a first person game.
I would also add that they could stand to tweak the art-style a bit, perhaps something more akin to what they're doing in Halo. Less swords and demons, more aliens and space-marines.
The controls could also be tweaked a bit to work better on the console in the new FPS viewpoint. Maybe tone down the whole class and skill tree elements and just focus on one main character.
And they should totally add some vehicles, maybe a bit of multiplayer game modes.
I like Halo a lot.