I wonder just what exactly they're gonna add though. Like I don't think adding more story acts is going to be that cool once you've done it 3-4 times and you figure you now need 3-5more hours of story leveling for every char, unless they add a separate leveling system(something that unlocks account wide after you've done it once like Scion maybe). It'll create a shit ton more models and tilesets and more variety and all that but making the leveling longer is fairly bad in general.
They will most likely add a new endgame stuff, considering it's teased here and there in the lore(The Elder serves a greater purpose for something, the Void or the Nothing etc) but that shouldn't take that much room.
I guess they are redoing animations and models of the main characters, so that's a bunch of stuff. Maybe they'll be redoing voices for all the act 6-10 stuff, using the occasion to recast some of the classes(which is sure to anger some people, I'm so used to the current Witch it'll be hard if they change it).
Anyway definitely excited about the exilecon stuff, although I have to check at what time it is, stupid NZ hours probably put that right in the middle of the night.