Alright, coming from a huge diablo fan here's my input. First off Bill Roper of Flagship (they have alot of the diablo 1/2 team) said that they pretty much will do another game like diablo but it probably wont be their first and i'm almost positive they've gotta make somthing new since they don't have the licence.
The diablo manuals have gotten progressivly worse. there is lots about the sin wars in the diablo manual and not to much in d2 and probably next to nothing in lod. The whole baal thing in act two has a bit to do with the sin wars and the horadrim(sp)
The diablo 1 manual had all sorts of stuff, all the prime evils and all the minor evils (Andariel and Duriel were minor evils) I think it even mentioned the world stone, it was just filled with goodies.
The Sin wars was the war between heaven and hell over the mortal world, its where the horadrim captured the greater evils in the soul stones. Diablo 1 begins a while after some evil guy lazerus(sp?) crams Diablo's soul stone into a kid prince, no idea why but he did.
Theres lots of room for a diablo prequil but it would also be pretty easy to mess up.
I like alot of you are waiting for diablo 3, in whatever form it takes.