Ace is dead and it should remain that way. As much as I like his character and as awesome as he was, Oda gave him an end worthy of his being. It's something a lot of mangaka's could take a lesson or two from. Killing a major character is already almost a taboo, but when it's done, often it's a character that is old or already served its purpose like Jiraiya, it's a character that 'can be missed' as it doesn't really have any potential left. But killing a young, dynamic, popular, awesome, cool character like Ace? That's unprecedented as it's a character that has almost everything to not make him be killed, still Oda did it an he did it in such a way you can't do anything else but like it. If someone would have told my 'Oda kills Ace' before I read it in the manga, I would think what load of crap would that be, but when you read, you just accept it. Especially when you compare it to Neji's death, which was quite blunt and random.
Bringing him back would just ruin one of the most epic scenes I ever read -.-