Why I think Ace will possibly die against Blackbeard.
Think about it. Bleackbeard is an awsome fighter, who could have injured shanks, without the powers of the dark dark fruit. Then Shanks tells Whitebeard that Ace doesn't stand a chance(not in exact words) against Blackbeard. That alone could foreshadow his death.
But the fight is in the past. Main characters in one piece only die in the past/flashbacks. So maybe Ace will die. If not, then what is the reason for having the fight 3 days in the past