Interesting, so there's going to be something new with Diablo III.
I have the feeling that the famous and probably hyped "Itemization Update" they were planning to do will end up being a feature of the expansion rather than being given in a patch for current D3, or maybe not, time will tell I guess. As for the lore at this point perhaps this time around we'll have to deal with corrupted or just plain pissed off Archangels, heck pretty much all of Heaven was invaded and desecrated by the end of D3 so there's got to be some "traces" or diabolic essence or shenanigans like that they could easily come up with. In fact I was totally expecting Tyreal to be a villain (at least temporarily, for instance due to manipulation or possession) in D3 since he was in the chamber at the end of D2 when the stone exploded, but instead he just temporarily became mortal and had to deal with amnesia, kind of anti-climatic based on my own expectations. Overall D3's story isn't exactly "great" anyway... it's rather decent at best, although I've seen much worse, but I have to say that Belial had a lot of [unexploited] potential.
It's good news nonetheless, Diablo III in and of itself screams incomplete in my book and needs to be worked some more, it could become great over time (I sincerely believe it has that potential).