Ace is not dead. No way.
At the very least we'll see him again after Ruffy beat BB. It's just a question of how long it will take BB to catch up to Ruffy.
I hope it deosn't happen in the near future, because I somehow have the feeling it would make the manga longer.
But Ace is alive. He told Ruffy they'll meet again, gave him that piece of paper and Ruffy told him that they'd had to fight over who gets to become pirate king.
That alone is proof that Ace won't die.
Ace dying would be the same like Whitebeard killing Shanks and Ruffy never being able to give him back his hat.
It's just not gonna happen.
If Oda wanted to go down that road he'd done it already (Lysopp would be dead)
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"