Quentyn existing entirely as a plot device to turn Dorne against Dany is one of Martin's worst decisions IMO. Especially in light of that last Arianne chapter in AFFC. It didn't make Quentyn sound like a badass or anything, but it made the plan sound amazing, and interesting.
Quentyn joining the sell sword company could have been very interesting. I was talking with my brother about this, trying to find a way for that arc to be more interesting. I like the idea of an ambitious Quentyn wanting to impress Dany and being very conscious of the fact that Gerris Drinkwater was more handsome and princely. I'd have them join the sell sword company and have a couple experiences with battle. During each Drinkwater would establish himself as a badass, gain the respect of the sell swords etc. Basically this would create jealousy and tension between the prince and Drinkwater, culminating in a Platoon type situation where Quentyn lets Drinkwater die during a small battle/raid.
The rest of the arc would then detail the journey to Meereen all while Quentyn suffers from PSTD. And by the time he reaches Dany he'd be a shell of the man he once was. Dany flirts with the idea of taking two husbands and asks Quentyn to join her army to prove his worth. He does, but as war inches closer and closer his fear of battle climaxes with his mad decision to free the dragons; that way it's not just a laughable attempt to tame dragons, but a desperate act of a man afraid to fight again (he believes the use of dragons would result in a quick win, basically).
Then have him get burned, as in the books, but survive long enough to bitterly send word to Dorne about Dany being mad/not worthy of marriage/a harlot/etc. Arianne gets the letter in TWOW and does what we assume she'll do (oppose Dany). Still a plot device...but far more interesting and somewhat tragic.