Sounds like a great idea, @flauwy. It would be nice to see a decentralized anonymous CAPCHA scheme being serviced by the under top 20 active listed witnesses for a small transaction fee from the reward pool. Maybe it is beyond the scope of your team's platform project, yet the link to a blog post of mine proposing it is provided for your reading pleasure.

https://steemit.com/steem/@novacadian/a-decentralized-capcha-annonymous-proof-of-brain-verification