Proof of Completing Challenges lol. It might work, might not work, but it would depend on having people solving the challenges. What about the nodes? Does each challenge solver have to run one, or can they join a pool?
How do you protect the system from 51% attacks? The challenge solvers might collude and hard fork the project.
We are considering to build the solution under NEO blockchain so I don't think we will have enough volume to over take the platform. So Challenge participants will just simply sign up the Challenge that got posted. Then when they complete, they are rewarded with tokens. They won't be having nodes nor voting power under this structure.
That being said, we have considered to build our own blockchain in the future. In fact, our co-founder is investigating the idea now as we suspect it may cost a lot of GAS for the community validation mechanism when we are scaling. We prefer to have registered businesses to be nodes instead just anybody. We are targeting specifically at small business owners, which mostly aren't techy/interested enough to coordinate a global 51% attack of a blockchain. Business owners will have to option pay tokens to post Challenges so should be in their interest to be a node as well.