Ace's last words:

Quote Originally Posted by Ace
If it weren't for the Sabo incident and a troublesome brother like you, I probably would not have wanted to live....
Now that Sabo has been formally introduced, I think we have a slightly better idea of what the Sabo incident could be. So what do you think?

I think Sabo was killed somehow, maybe trying to save Luffy, and his dying words were telling Ace to live out their dream and become a Great Pirate and to protect Luffy.

Regardless, let's try to do the impossible, try to predict Oda.