I want any feature, in this case Coding Challenges, that is not directly related to asking or answering questions about specific programming problems, not to affect the Q&A reputation score, and not to grant access to Q&A moderation privileges. Otherwise, the meaning of the reputation score will be less about its original purpose and might become less relevant.
The focus should be on helping new and old users to grow as programmers and to develop skills that will help them to write good questions and answers, rather than on earning reputation points for not asking and answering questions.
Suppose there are challenges based on unanswered questions. In that case, the reputation earned, if any, should be based on the upvotes or downvotes received for answering the question, not for participating in the challenge.
Consider creating an alternative score system and decoupling the privileges related to accessing Meta, the chat system (chat.stackoverflow, chat.stackexchange, and chat.meta.stackexchange), and other experimental features, like Discussions, from the Q&A reputation score.