Based on
the criteria reveiled:
Unable to permit disharmony,
you will be disappointed by fate,
and lose sight of true strength...
Misreading the truth,
you will venture forth in secrecy...
Aced seemed to be the most likely one to be the traitor based on the final KH X update. But based on the added information given from the TGS 2016 traitor, he only fits two out of five.
Unable to permit disharmony: Before the war started, Aced seemed to be willing to work with the other foretellers to help prevent the war. So I think all the foretellers fit this criteria.
You will be disappointed by fate: This could be Aced being disappointed by how the other foretellers are handling the growing tension of the keyblade war. Though this probably is not what this criterion is meaning.
and lose sight of true strength: I'm unsure about this one as it could be Aced forming an army of soldiers as the keyblade war draws closer.
Misreading the truth: Not sure about this one either but it could mean that Aced misread what the appropriate action to take to prevent the keyblade war.
You will venture forth in secrecy: Aced didn't seem to be very secretive, He openly speaks his mind, examples include: saying that he would defy his masters teachings to Invy and trying to make an alliance with Ava right after knocking out Gula (I think he was still holding his keyblade too).
Ava is another close candidate for the traitor but she still doesn't conclusively fit all the criteria.
Unable to permit disharmony: All the foretellers seem to fit this criteria.
You will be disappointed by fate: Ava was disappointed by the fate of the keyblade war and so created the dandelions and tried to reason with the other foretellers.
and lose sight of true strength: This one could be her behaviour in the final KH X update during the keyblade war.
Misreading the truth: This could be to do with what Luxu told Ava which lead to her behaviour during the keyblade war.
You will venture forth in secrecy: Ava was probably originally secretive about the dandelions. Other than that I don't recall remembering anything else she was secretive about, in fact she seemed to make an effort to talk to the foretellers to avoid the war.
The criteria for Gula:
Unable to permit disharmony: All the foretellers seem to fit this criteria.
You will be disappointed by fate: This one is uncertain.
and lose sight of true strength: This could be Gula giving up on trying to stop the keyblade war and fell into depression.
Misreading the truth: Unlike the other foretellers, Gula knew what was on the lost page. He tried to find the traitor but seemed to have followed the wrong clue.
You will venture forth in secrecy: Gula seemed to not tell the other foretellers about what was on the lost page.
The criteria for Invi:
Unable to permit disharmony: All the foretellers seem to fit this criteria.
You will be disappointed by fate: We don't know enough about Invi to prove or disprove this criterion.
and lose sight of true strength: We don't know enough about Invi to prove or disprove this criterion.
Misreading the truth: We don't know enough about Invi to prove or disprove this criterion.
You will venture forth in secrecy: From what we have seen in trailers and KH X; Invi seems to be very secretive. But she does seem to talk with Ira.
The criteria for Ira:
Unable to permit disharmony: All the foretellers seem to fit this criteria.
You will be disappointed by fate: This probably attributes to Ira being trying to convince the master of masters to help stop the keyblade war as well as accusing the other foretellers (I think he was accusing Gula due to Gula's body language).
and lose sight of true strength: Ira lost hope and his reasoning for fighting in the keyblade war was so nobody wins the war.
Misreading the truth: We don't know enough about Invi to prove or disprove this criterion.
You will venture forth in secrecy: From what we have seen Ira doesn't seem to be secretive.
I might have missed something or got something wrong since this is from memory. But out of the foretellers the least likely ones of being the traitor are Aced (Ironically) and Ira. We don't know enough about Invi to accuse her of being the traitor or not. Ava (unintentionally) fits most of the criteria of being the traitor. Gula is an unknown, as most of his scenes in the trailers and KHX show him in a depressed state. This makes me skeptical to call him a traitor of not.
But its important to note that all the foretellers fit at least one of the criteria. This could mean that they all betrayed each other and so all of them are 'the' traitor.
The Master of Masters could also be the traitor as he also fits some of the criteria. Luxu is an unknown as we don't know enough about him or his motives as he is following the Master or Masters orders.
All criticism on this analysis is appreciated