In gamification design, we often host a design thinking workshop, which generates deep discussion typically on why certain behaviours should be encouraged or discouraged, features added or not and much more. Design thinking discussions require some element of open mindedness from all participants.
Each group will have some heavily engaged and participative members and a each team will have maybe one or two nay sayers. It is always a challenge to break an impasse when either side begins digging in their heels. My view typically is that we break the impasse by looking at the data or to look for data with an a/b test. In an A/B test you randomly give the users one of your two options, any user will see only one and you look for data on the behaviour you are trying to test and confirm. The version that leads to the desired behaviour the most is what you eventually choose for your design.
When you are in the early design phases obviously that data may be missing. You may not have a product yet, etc. What you hopefully are clear about is the target audience you are aiming for. My suggestion would be to then to create a paper or mock-up version of your two options and look for feedback from the target market.
In the design workshop settings, I often use the voting technique or ranking where each individual has one vote for their favourite or we rank their top three. It helps focus the mind and break an endless debate. The key is always to realise that reality based on real users using your gamified experiences will be the true acid test.