We have a cowboy coder in our team i.e. he likes to take advantage developing some new features or tools that are not always requested nor useful. We use scrum/kanban for development, and it is quite common that when he gets an item from the board, he takes a long time to finish it and comes out it the solution and "something more". Now thinking by the human side, I always try to fit someone to do what he/she likes the most, so I would like some advices on how to make him feel motivated doing the stuff he likes, at the same time that he delivers useful features and share his "something more" stuff development with the team.