Board Thread:Questions and Answers/@comment-25371179-20160627111506/@comment-26271908-20160703010743

I don't think they ever planned to place Trystane in King's Landing, anyway. Personally, I think they were trying to find a new storyline for it, but the lack of story to Dorne would need them to push it into other stories until they could bring Daenerys, and they had sufficient storytelling for the season, so they killed him and Doran off to finish the Dornish plot