Broaden weapon selection, these are versatile fighters, why are they so incompetent with anything other than their archetypal weapon sets? A barbarian should in theory understand bows/xbows. a demon hunter should have a few abilities for one handed swords, monks should understand more about throwing things....
Allow for offline gameplay, if the restriction is needed, then don't allow for players to use the same toons online as offline.
up the curve of normal->nightmare and nightmare->hell so that it's actually a bit of an accomplishment to see it happen.
Allow for a checkbox (Auto-off on replays/ auto-on on first plays) that turns off cinematics and speeches.
As for lore? I want to know what happened to the heroes from d2, one of the best things about d2 was how it so perfectly wrapped up d1, the sorceress I would imagine to still be alive, the Amazon, assassin, and druid also were young enough to live so long. the necromancer.. .was he Nilathak? or will he come back? did the darkness consume him or did he hold on to his sanity?
Where is the Paladin in all of this?
Did they all fall to darkness? are they the newest lords of hell awoken from a slumber into madness?
Could you add two new quests to freeing their souls
DO YOU EVEN REMEMBER THE LINE "Rest well.. Blood Raven"
How about a more comprehensive hireling system? they only add more to customization, even just upgrading followers to be viable at later levels via flat effectiveness of stats and abilities being increased.
How about something as simple as "Cathederal Gauntlet" in which the players go through the game of diablo 1 as quickly as they can, gaining better gear for %complete and speed of completion, there weren't all that many quests in D1, it could be as simple as finding/touching a shard of Diablo's soul stone and being somehow linked (By fate maybe?) to his memories. even a curse that tries to resurrect him within the darkness of your own character by putting you in a PTSD-like trance to remember his exploits.
More quests would be nice but they aren't the biggest thing