Doran didn't send Quentyn to Meereen

[Prince Yourwetdream Aeryn]

If Quentyn was kept hostage by Yronwoods why does Doran send Trystane to King's Landing? 

[Maester Saramello]

He wasn't so much a hostage as a "guest." He was in no danger of dying, Doran would burn Yronwood to the ground if they had harmed Quentyn. It's simply a matter of Yronwoods having influence over Quentyn later in life as payment for the loss of one of their own. A tutoring of sorts if you will. Also, Arianne is technically the heir to Dorne. 

 

It's to get influence over the youth. People often have strong fealings towards those who fostered them and kept them as wards(Robert and Ned for example). 

[Ralphis Baratheon]

I think Doran was conflicted all along on who he wanted to rule Sunspear or be Queen or King consort. So he settled it in by having a contest. Who will be able to ride a "dragon" first Quentyn or Arianne? My bets are on Arianne and her magical nipples.

[Maester Saramello]

The original plan, as laid out in ADWD, if as follows(before sh*t hit the fan).

Viserys marries Arriane and they become king and queen of the seven kingdoms.

Daenerys marries Khal Drogo, brings army for fighting, and after Khal dies(which he likely will fighting 7 kingdoms at once) she's pre-betrothed to Quentyn just in case, further solidifying the alliance. 

Aegon(I think)- I am fairly convinced that this guy is not only fake, but is a backup plan if viserys died. He would essentially take up Visrys's role and marry Arriane(until he got too air headed.) If he is a blackfyre(or brightfire), added icing and another layer on the cake. 

On a side note, I'm convinced Varys will kill Mace eventually. He's an old fart who surrendered an army of 40k men after the battle of the trident, the reach needs a more martial ruler loyal to the crown(Willas/Garlan/Randyll Tarly).