The People Closest Know Best
The same is true with MVP programs. Any metric you establish as criteria for star participants will cause unintended side effects.
Members might answer the easiest questions, create fake accounts to vote their own ratings up, or get others to create questions they can definitively answer.
Then when you adapt the standards to prevent this problem, you’ll cause an uproar.
You can avoid this pretty easily.
1) Set up multiple journeys (or quests). Not everyone is meant to be the most active member. Some (most) have a specific topic expertise, experience, or their own good intentions to share. Let members pursue different pathways based upon what they want to do. Or…
2) Give community managers full discretion. Let those closest to the community decide who gets a badge and why. Let them create new badges for new situations. They know your members better than you will ever do.
A badge should reflect the contributions each member makes to the community. Since every member makes different types of contributions, you should have plenty of different badges.