just pondering some ripples.....
AU core change: Lucius was Imperiused for the duration of the first Wizarding War.
-> Draco was not born out of love, but at best coercion, and at worst horrific abuse.
-> Lucius doesn't want anything to do with "his" family and is constantly uneasy around them, so is very distant.
-> Narcissa could go either way, but if we go with the frankly more dramatic "the child is an unpleasant reminder" route, she would have zero compunction cutting herself off from him and basically having nothing to do with her "family".
-> The raising of the child is left almost exclusively to the house elf, creating a more "Regulus & Kreacher" situation.
Dobby:*after slamming his fingers in a door because one of the parents was mad he was late with dinner* Dobby longs to be free... :(
Draco: *head on his arms, looking out the window of the top floor room where he lives* Yeah, Draco too.
-> Draco wouldn't have spent much time around people. He'd have well-paid tutors, of course, but other than that he'd be isolated. Spending his whole life alone in the manor with occasionally Dobby for company. Sent out to do his school shopping by himself. Probably tried acting out for attention when he was little, but by school age is completely trying to be perfect and self-sufficient.
-> Instead of being a cocky, social, extroverted showoff, first-year Draco is a withdrawn, serious, lonely semi-pariah. (Having previously committed to abusive Imperiused Lucius for the backstory, it follows he would have been known as a brutal Death Eater with horrific crimes to his name, and it can be assumed most Wizards would still assume he was lying. Including, perhaps, the children of the other Death Eaters [i.e. his whole dorm except Blaise, who is an asshole anyway], whom he wants nothing to do with.) Probably doesn't know the last time he smiled.
---> Dumbledore as a Legilimens is aware that Lucius is telling the truth, and testimony like that is why he's fully pardoned, but that's not going to stop the rumor mill. Most people probably just assume he bribed his way free, if anything.
-> Instead of the "wrong sort" conversation with Harry in Madam Malkin's, Harry and Ron see Draco shopping on his own and Ron spills the exposition about his evil family and how Slytherins are bad.
Things which don't change: Draco's still a "the Hat doesn't even have to touch his head" Slytherin, and he's still desperately needy for approval. It's just that now instead of it being because he idolises his parents it's because he's got a huge gaping hole which is currently filled with little more than internalised self-loathing.
Draco: You don't understand anything! My parents hate me! I should never have been born! *sobs*
He's also probably still mean as hell. But not in the mean-girl-social-bullying kind of way like normal, but in a defensive and thoroughly vicious way to keep people away. Full on mistrustful hedgehog. Lack of socialisation means he doesn't talk to people except to push them away.
-> Draco doesn't teacher's-pet with Snape. He wants nothing to do with him, just like all the Death Eaters' kids.
-> Not that I'm saying Draco would get to the Mirror of Erised, but if he did, and his vision was a happy family? Man, that's just heartbreaking.
Year 2:
Basically the whole plot of Chamber of Secrets falls apart a bit if Lucius isn't playing the role of moustache-twirling villain...
-> Lucius would have no reason to send the Diary Horcrux to Hogwarts. In fact, he might very well not even be aware of what he has (even less aware than canonically). I can definitely see it still being with him, having either been entrusted to his father and then inherited, or else given to him under the Imperius to be vaulted for safekeeping.
-> Dobby would have no reason to try to keep Harry Potter away from Hogwarts. Or know Harry Potter at all.
-> Say Lucius did have it. Perhaps he discovered it and tasked Dobby with disposing of it. Or, perhaps Draco found it himself. An isolated loner kid who has a lot of problems, who finds a supportive sentient journal that wants to listen? He'd be tantalising fresh meat.
tl;dr this AU sounds like a good excuse to write sad Draco with the Chamber of Secrets diary and coming to the "I don't actually want to kill people" epiphany when he's 12, not 17
..........About this time is when I get distracted.