Hey Heath, why don't you imply a time limit on challenges?
Here is what i mean. This rule if from the ride game on TR.
• In the event a challenge has stalled the game, 36 hours will be the limit of it's duration. After that time limit the person who suggested it can either do it themselves or suggest a new next challenge. If they don't reply in 12 hours then the previous winner gets to pick the new next challenge.