I DO BELIEVE THAT ACE IS ALIVE:-

Of course there aren't any proof yet,only theories. Some years ago I found a website where people were talking about the fact that the chapter "the death of Portgas D. Ace" was a metaphor more or less:

I think he never died at Marineford, that shanks somehow saved him and will return with his real name Gol D. Ace. Afterall ODA never confirmed his death, he was always so ambiguous, and in an interview he said that when saw the anime episode where Ace "died" he got really angry they made it too sad and harsh, I wonder why...

All people who say "VIVRE WAS BURNED! SABO ATE HIS FRUIT! ODA DIDNT BRING PEOPLE BACK TO LIFE!

Why are you so nervous? Remember we are talking about Oda, and I think he has been keeping trolling us all this time...

Or

I think his devil fruit is the exact reason why he could possibly be alive, We know that s Mera Mera no Mi is powerful, but what if....it AWAKENED? With his fire already being so strong, the only thing that could make the Mera Mera no Mi stronger in its Awakening form is.. Amaterasu, Amaterasu is a Goddess of Sun and a part of Japanese mythology, Some other animes/shows took Amaterasu for their plot, for example Naruto, In Naruto's case Amaterasu is a Black flame attack, and this is what I think Ace's awakening would be like, Amaterasu produces black flames which are able to burn anything, including fire itself, I Think this would fit the Mera Mera no Mi awakening but that's not all there is Amaterasu, The black fire of Amaterasu can't be extinguished, the only way it can be put out is if the user puts it out, I'm not saying Ace's awakening will be Amaterasu itself because, you know, legal issues but it will be something like Amaterasu, it doesn't even have to be black, but his awakening has fire that can't be extinguished. This is where l come back to Ace being alive, I'm going to assume Ace managed to do the awakening since he had a bounty of 550 million and was the 2nd division Commander of the whitebeard pirates, meaning he was very strong, if Ace used his awakening at least once, there would be a way to make him alive again, But it would take a genius to do something like that. That genius IS.... Vegapunk, Known as the most intelligent person in One Piece by far, I think he's capable of reviving Ace, We've seen Ace dead, but his awakening flame would still continue burning even after his death, All Vegapunk would have to do is take a sample of his awakening fire and with that sample, he would be able to revive Ace completely, I believe this because of everything he managed to do with Kuma, He obviously knows what's he doing.