Smartwatches, which can monitor users' health data and encourage healthy habits, have gained great popularity in recent years. Meanwhile, as a useful design method to improve user engagement and motivation, gamification has been widely used in fitness applications on smartwatches. While existing gamification research suggests that users have different preferences for gamification elements in different contexts, few studies have examined users' preferences for gamification design on smartwatches. Therefore, this study sought to obtain detailed opinions and suggestions on smartwatches and gamification through semi-structured interviews of sixteen current users. User preferences for sixteen different gamification elements related to the development of fitness applications on smartwatches were investigated. This study concludes that users prefer overviews, goals, challenges and rewards, and dislike time pressure. Our results also show that different users have different preferences for gamification elements. This qualitative study has formed an important basis for an ongoing quantitative study on tailored gamification, which further proves the need to classify smartwatch users based on their preferences and characteristics, for designing more effective gamified fitness applications on smartwatches.