Thank you for clarifying, Martin. I have no direct answer, I will have to think about this, but the dampening sounds like a good (temporary?) solution.
Somekind of a voting system between the players before the time starts to run could be interesting. 
maybe hard to implementate though.
The scenario could be the average of the votes. 
What would be neat is maybe set “priority” objectives. Goals are set for what’s important for the challenge, any restrictions, and then the proper range/limits are set by the system automatically. That way what’s important or what should be a goal can be set without having a preset “win-all.”