Ok so this theory is semi-crackpot, but I feel like it has a tendril of truth in it.
So everyone remembers Gerold Dayne, aka Darkstar. He is a knight of House Dayne and a renowned fighter. His most famous act is "helping" Arianner Martell with her plot to place Myrcella Baratheon on the throne, but when that went sour we tried to kill the little princess, succeeding in only cutting off her ear.
However, we get almost no history on Darkstar. No parents, no motives, nothing at all. We don't even get a good description of him: in his 20's and handsome. (remember his age, that is somewhat important to this theory.
My theory is that he is the bastard son of Arthur Dayne. Arthur Dayne, aka the Sword of the Morning, is of course the famous knight of the Kings Guard who Ned Stark kills at the Tower of Joy (more on that in a moment). To be honest, I have no idea who Darkstar's mother would be. Other than Ashara, Arthur is the only Dayne who is really mentioned in the series. And Darkstar can't be the son of Ashara, I it would have been mentioned somewhere(right?). Plus I'm pretty sure that Ashara's stillborn (?) child was her first.
Even if Darkstar isn't the son of Arthur Dayne, then this next part serves to tie him to Arthur Dayne. My idea is that he was a squire to our favorite Sword of the Morning. If I'm not mistaken this is a popular theory. So if Darkstar is Arthur's squire, then he would have been at the Tower of Joy with the other King's Guard members.
At the beginning of Game of Thrones, Jon Snow is 14 years old. That means the rebellion finished around 14 years ago. So if Arianne Martell's description of Darkstar is correct, we can assume he was about 8-15 years old at the Tower of Joy. This is the perfect age for a squire, especially if he was related to Arthur Dayne.
So obviously Darkstar survived the Tower of Joy, as he was just a squire. He would have watched the knight he serves die at the hand of Ned Stark. But he also would have been witness to the birth of Lyanna's child (Jon right?). Even if Darkstar wasn't in the room when Ned Stark was with Lyanna, he could have placed 2 and 2 together. Before Ned came there was a baby. After he left there was no baby.
At some point Doran Martell states that "Darkstar is the most dangerous man in Dorne." I am inclined to think that he is dangerous not because of his fighting prowess, which is impressive, but because of something he knows. This thing that he knows is probably that a Targaryen child is with Ned Stark.
Now of course Darkstar doesn't know what the child's name is, or where the child is now, but he knows the true heir to the Iron Throne (if you think Aegon is truly dead) is with Ned Stark. Eventually, as he grew up and heard tales from the North, he probably could have pieced it together that Ned Stark is passing the heir to the Iron Throne off as his own bastard, Jon.
The reason Doran thinks Darkstar is the most dangerous man in Dorne is because Doran knows that Darkstar knows what went down at ToJ. What I can't figure out is if Darkstar told anybody. I don't think he told Doran, or we would have some hint of it in his POV chapters. (It wouldn't be smart for Dorne to support Jon's claim to the throne right now anyways because who is going to believe that Jon is actually a Targaryen. No one else really has any idea of what happened at ToJ.)
This doesn, however, explain why he tries to kill Myrcella Baratheon. Eventually he wants to place Jon on the throne, as he serves the Targaryens like his father. It is my opinion that he meant to kill Myrcella all along, and when their group ran into trouble he seized his opportunity.
Maybe he is currently traveling north to tell Jon the truth he knows. This could be the big unveiling that we wanted.