My wild guesses on the next (and hopefully NOT last) expansion:
New class: Bard (INT based) (why: remember the Bard in Diablo Hellfire? we also had the Monk and the Barbarian back then...)
New Act: Act 6 will be split up in 3 pieces: We're going to see Duncraig and Kingsport, followed by Skovos Islands and finally we're going to be in the burning hells again (Chaos Sanctuary, anyone?) Another wild guess would be the Jungle (maybe the home of the WD)
New feature: Never-ending Dungeon; with random quests, challenges and bosses inside. Once you die, you'll have to start again at the very beginning. (group play highly recommended)
We will probably face some of the Evils again. Maybe first boss is Andariel + Duriel as a pair, a corrupted Tyrael (corrupted by the Souls of Belial and Azmodan) and finally "The Evil Trinity" - all three brothers at once, but weakened. They manage to escape tho and we gotta face them in the next expansion.
From my look at it, the bard wasn't official lore. I like the feature idea, but as for fighting duriel again? How would that happen, I can see Andariel, but fighting all brothers at once, that would be hard to see.