I have actually been quite curious on this matter. My biggest pondering is that of the name Aidan. After completing the quest you mention, the player receives an excerpt from Adria's journal discussing Aidan coming to her with a troubled heart and her 'consoling' him as best she could. She then mentions that wherever he goes hell is surely to follow. After some research Aidan is never mentioned before D3, however he is mentioned a couple times in D3 as the son of the monarch and the original slayer of the Skeleton King (according to Diablo Wiki). Interestingly enough the players are the original slayers of the Skeleton King and according to the Lore, the warrior is the one who actually completes the deed, going all the way through the dungeon and becoming the Dark Wanderer after slaying Diablo and inserting Diablo's Soulstone into his skull.
So if this all adds up, the writers are finally giving a name to the Dark Wanderer and his name is Aidan. This all kind of fits your love child theory as well because if, say, he had just slain Diablo, he would have the Soulstone in his skull, then he would go and see Adria, who then would 'console'(aka sex) him as best she could (she says he has a troubled heart, probably because of his internal struggle with Diablo), then he would leave (and since he is becoming Diablo, the statement "Hell would certainly follow" would make a lot more sense). She would become pregnant giving birth to a girl Leah who would then, theoretically, have some part of Diablo in her even if it is just a small part, due to the Warrior beginning to become corrupted by the Soulstone. (although I have no idea what having a small part of Diablo in her would possibly signify, just saying that some part would most likely be there since Diablo loves himself some corruption)
All of this fits the 20 year timeline like you said and leaves some interesting role for Leah to fill. Of course this is all speculation and theories with more holes then swiss cheese, but, if you look deeply enough they all make sense and sets up the story nicely for possibly some corruption of Leah or some betrayal by Adria.
The only issue I have is the resetting of some of the Lore. I mean essentially we are just finding out more information, but if they are giving the Dark Wanderer a name, that both nullifys the fact that the player's Warrior from D1 days were their own creation, and in actuality was just another character that became a plot point. This kind of takes away from the Diablo Trilogy theme of you being the hero by creating your avatar of sorts, but really this is just a minor complaint that i wouldn't care too much about if the story is well written with intrigue and plot twists. All hail M. Night Shyamalan :P
Edited by Bauldur on 10/5/2011 9:57 PM PDT