The most notable omission from Game of Thrones has only actually appeared in one novel, A Dance With Dragons, where he meets an unceremonious end towards the book's conclusion without really accomplishing anything. Nevertheless, Quentyn Martell's quest is quite extensive, and sees him journey to Daenerys' at his father Doran's behest, given the latter's closeted support of the Targaryen cause.
This journey is a long one that involves a number of additional characters that have joined Quentyn in not making the cut on screen, including his most notable companion Gerris Drinkwater, who collects 115 mentions of his own. His group encounter bandits and infiltrate Meereen's naval blockade without contributing anything to the overarching plot, whilst his eventual arrival results in a swift dismissal by his target. His subsequent death puts an end to a narrative that seems to serve no purpose other than padding the novel's length.
Doran was given just the one son on screen - Trystane - who wasn't fleshed out at all before his assassination at the hands of the Sand Snakes. Trystane is but a child in the books, where Quentyn is the middle child sandwiched between him and Arianne, the latter of whom should've been the one included if the series' creators were determined to condense the Prince's three children into one, even if Quentyn was technically the most prominent.