What I'd love to see in D3 is having the ability to stray from the storyline. Clearly it's not easy to implement, as there's a basic path in the game that must be followed (ie Act 1 -> 2 -> .. -> 5), but perhaps one could deviate in the penultimate Act, and have a slightly different last Act experience.
What I mean by this "deviation" is the ability to do exercise free will, instead of simply carrying on being the "warrior of light" that D1/D2 forced you to be. For example, Tyrael destroying the Worldstone is clearly disconcerting, and the Harrogathians must be talked to about it before going ahead and doing it. Why not offer the ability to stop Tyrael, even if that means having to battle him, and perhaps even kill him? Perhaps you could ally yourself with Diablo, for your own nefarious reasons, or simply because you don't think the "forces of light" could win. Or just because you despise the NPCs, who, despite relying on you to save them, are charging you obscene amounts of money for their goods. Let's face it, who wants to save the Rogues? You'd risk your life for Kashya? Charsi? Gheed?!
It'd also fit in with the "dark fantasy" theme of the series. Despite the stygian, brooding atmosphere, the player is immensely "good" -- the most they stray from being kitten-worshipping Mr. Roger clones is some choice snippets like "All who oppose me -- beware!"
D3 has the Barbarian -- tired, lost his purpose in life; the WD, a weirdo; the Wizard, a rebel; and the Monk, who, while is described as "holy," does what he thinks would be the best option -- he, in my opinion, wouldn't mind stopping Tyrael from destroying the Worldstone at all costs.
So, for those who'd like a tl;dr version of my disjointed thoughts:
- Ability to stray from being "good" -- not necessarily just pure evil (ie join Diablo and skewer Deckard Cain, as tempting as that sounds), but a few important options to deviate from the "holier-than-thou" storyline and committing neutral/evil acts (stop Tyrael from destroying the Worldstone, talk back/attack annoying NPCs like Kashya and Nihlathak), and ultimately getting an alternate cutscene (for D2, I'd imagine something along the lines of the player sitting in Baal's throne, looking evil-ish and smiling, while Tyrael lies dead in front of him)