With that being said, I would like to see the demons win during the expansion. If it's done right, it would certainly bring a lot of darker tones, and a grim atmosphere to the game. And to be frank, it would be different than the past installments in the series.
DI - Game starts off with Diablo in control of Archbishop Lazarus, King Leoric (through the archbishop), and Leoric's son Albrecht. The game is really dark, gory, and not even the safe haven of Tristram feels that safe. However, despite a dark narration at the end of the game, the side of good wins... sort of.
DII - Game starts off with you chasing your former character from DI, which already has a sense of loss and disadvantage, and eventually all of the Prime Evils are released on the mortal realm and the game really feels kick !@# at this point. The game is also dark, and there is plenty of blood, demonic sacrificial areas, and the game never gets too cartoony - LoD's cinematic with Baal at the gates toying with the lone guard is a priceless cinematic! However, in the end, the side of good wins at a price.
DIII - Cain dies (long overdue honestly), but Tyrael ascends back to Heaven as the aspect of Wisdom as a man - not an angel - Legion is defeated, the Heavens are returned to pristine glory, and there are no major demons (supposedly) left to rule over Hell. In short, the side of good wins big time... with the exception of losing Leah.
For the expansion, why not bring up a few loose ends? Say... bring Malthael back as the main antagonist? He was the Angel of Wisdom, why not make him the Fallen Angel of Chaos or something? He fell from grace in Heaven, and was supposedly driven insane at the destruction of the World Stone, so why not corrupt him over time, as he is no longer able to gather strength from the Crystal Arch, and cause him to reject all the wisdom he once cherished and take it all for lies and deceit? After all, he's been walking the realms attempting to find answers and understanding due to the events that took place during DII and before DII - sounds like a perfect time to whittle away at the will of a weakened angel. While we're at it, why not give him a seat of power over Hell... after all, (supposedly) all the Prime and Lesser Evils are either dead or otherwise unable to rule over Hell. Further, he's the ONLY member of the Angiris Council that we don't see in DIII. And because Malthael was a member of the Angiris Council, and the Angel of Wisdom, perhaps he knows a way or has access to a heavenly tool that could restore the Prime and Lesser Evils? If you paid attention to the game's last cinematic you would have seen the Black Soulstone still intact falling from the Heavens.
Think about it, there is no longer an Angel of Justice and the Angiris Council is broken and incomplete with a human member among its ranks... who's to stop Imperius from performing acts of grave injustice through his arrogance and pride - who's to stop him from going back and destroying humanity because of its angel/demon blood? If you think about it, if Tyrael hadn't stopped Imperius from destroying humanity... there wouldn't have been any nephilim left to defeat Legion. And if you bring Malthael back as a leader of Hell, now you have a direct connection between the leaders of Heaven and the leaders of Hell and it makes it far more personal for both sides.