The time it will take before we will see a challenge mission implemented to the game is somewhere in the ballpark of infinity. But why wait when you can just use the forum. I'm going to post a new topic where challenge missions can be played out. For anyone who wants to try out the challenge, post a message under this topic that you want to participate.
The Challenge can last for multiple turns and the state of the board and the moves that the participant makes should be marked as a spoiler to avoid spoiling the solution for other participants.
How it plays out is that I will post the state of the game and put at the top the player that is taking the challenge and what the current turn is. I will then update the post and the turn number so it will be easy for the participant see the update. The one participating should put at the top the number for the move he is making. Then the participant updates the post and the number for his move to make it easy to see the update. this will continue until the participant wins or loses.
So anyone up for a challenge?