Yet another tragedy of Ace's death is that he and Sabo not only never get to reunite but they never really get to know each other either. They didn't get to see each other grow up and become their true selves.
To Ace, Sabo will always be that ten-year-old dreaming of writing an adventure book and navigating a pirate ship. And to Sabo, Ace will always be that ten-year-old angry at the world and stuck in the shadow of a father that he hates. They are ghosts to one another.
Ace will never get to see Sabo be a freedom fighter taking on the World Government and saving people. Sabo never got to see Ace find a father figure that he could be proud of and look up to. They never get to see each other fulfill their dreams. Never got to see each other change and grow.
I always imagine if Sabo meets more people that knew adult Ace, like Marco or Deuce for example, and they tell him how polite and kind Ace was, that it would be kind of a shock to Sabo. Because a kind and polite Ace is someone Sabo never got to experience. Would he even believe them? "Ace with manors? No way!" Maybe Luffy has to explain to Sabo just how much Ace changed after Sabo "died." And Sabo just has to live with the fact that his older brother only became nice after they thought Sabo died.
There's a pain there that will never fully go away. A pain of never really knowing someone that you love dearly. Knowing that you love them but also knowing that they're a stranger to you. Sabo's pain of having other people tell him how his brother - his first ever friend - grew into a wonderful person that Sabo himself never got to see and never will get to see. Ace's pain of thinking that Sabo's dreams died with him because of the horrors of the nobility - Sabo's own biological family - and the Celestial Dragons took away his freedom but Ace will never get to see Sabo fighting to protect other people's freedom from the same oppressive system that tore them apart.
They both grew and changed so much. They would be so proud of each other.
But in their memories, they will always only be a dead little boy to the other.