Ace is the worst. That was the most emotional death of a beloved character, brother of the MC, and also was the middle milestone of the whole opera. There was also the meaning of giving the torch to Luffy, inherited will and all. That was one of the most important moments in the manga, if not the most, and probably the most iconic. The climax of Marineford which is the climax of One piece. End of the wank.
Ace coming back would be the biggest middle finger Oda could throw to his audience, and it is so big that for sure Oda, even if he has fallen from grace, will never be so stupid to actually do it.