For the expansion, I think the inevitable battle between Order and Chaos should be called into play, with Necromancers and Paladins (Templars, whatever) being the two new classes. This would also allow Skulls and Diamonds to be brought back and represent each of the newer classes in socketing...later on past this expansion, if they manage to garner enough success, they could, in theory, do a third expansion that brings the Druid back...but back to the idea for the next expansion...
A large majority of people enjoyed Diablo 2...they hold it up with high praise, and anyone can argue its true value, but regardless, I think people would enjoy a second round at Mephisto and Baal, and they could do Diablo proper, but maybe with some sort of twist...I have no idea. This time jumping would also allow Imperius to come back into play...whether or not he dies at the end of D3, and he could theoretically be the primary villain by merging with Diablo, draining him of his evil, taking his place, and what not. I imagine the idea of Tyrael taking Imperius' spot would mirror Imperius taking Diablo's spot.
We would be allowed to have good ol' Deckard Cain back (would be cool if he could be the artisan for Socket Runes, or perhaps he holds the Horadric Cube and transmutes items for you)...perhaps we could intervene in some crucial events throughout 1, 2, or 3 that we may or may not have seen along the way. For example, we might be able to 'save' Deckard Cain in 3, or we might be able to kill Adria in 1...this might screw stuff up, and make things worse or unexpectedly bad towards the end (Multiple Endings? Good Ending, Bad Ending, etc.) Just enough difference to make it separate from the previous titles. Maybe we can save Wirt or Griswald? They would make for some funny dialogue. Or not...I dont care.
Player Housing was a topic I heard mentioned before (the poster's name escapes me) but I thought that was an outstanding idea. Whether or not it just be a shared room in each town, or a "Teleport To" portal where you can invite friends to chill and see achievements, and so on. This is going off on a slight off topic tangent, but I think to fill up furniture should be based on Deeds like Ultima Online has, where you use the deed to place a permanent item in your house, then you can break it down and move it from deed to item. To get the item would be achievement based, rare vendor based, or perhaps even from breaking the environment. So say you break an alchemist table...that table might have a 5% to drop a deed for an alchemist table. The uses for the furniture could be solely cosmetic, or certain pieces could have functions like a forge, crafting, or whatever else. Just a thought.
And lastly we have the Secret Level. The infamous Cow Level could be brought back and improved 100x in size and scape. Cows could be varied in appearance, the Cow King could be an epic end game fight that sits upon his throne of broken barn fragments, and to make things even more unexpected, to access this Secret Level, we might have to do the rumored way in the 1st Diablo to access it...which was only a rumor and totally escapes my mind at the moment. Otherwise, put back in the Horadric Cube but substitute Wirt's Original Leg from Diablo 3...bingo.
And that's all I have...I doubt any of this will be touched upon, but I figured if I got my thoughts out and they end up using some of it, awesome...I'm also sorry if someone else on other posts have mentioned several of these things...I have read a few and I dont think I saw any of the above except for the obvious things. Anyways, good times ahead (I hope), thanks for reading it (especially if any blues are), and feel free to let me know what you think.
Good or bad...I am going to grab some of that new black rum by Cpt. Morgan.
PS: Also bring back the Horadric Cube and make some new recipes that I can use to salvage junk items to make better (fixed) items, or at the very least new crafting ingredients.