There's a promo that said "would you know if your brother came back wrong?" and it has sammy looking scared and dean in the background. so Dean might be a lil' evil when he gets out.
Kripke said in tv guide that Dean's been in hell for four months before sam got him out but four months is alot longer in hell time than it is in our time. So Dean Spent some time down there.
I have a theory. They never said where Sam went when he died.Ever. They never said he went to hell. So what if he went to heaven and because he was refusing his destiny he was given like "divine" powers? Like he was able to keep his powers but use 'em for good? And if Dean comes back evil then we could see Dean on hells side and Sam on the good side (our side). That would put a huge twist on the show that no one would see coming. Everyone says Sam is evil and what if he's not and he fights Dean who is evil?