I spent a significant amount of time since last year working on a decentralized ad protocol called Native Ads. Read more here: https://github.com/imwatsi/hivemind/blob/master/docs/native_ads/native_ads.md
It uses time and space as metrics to monetize. Supports burn payments as well as send to account payments.
Originally, I had tied it to Hivemind and implemented it in my fork of Hivemind called Hivemind X.
Now I am currently in the process of separating its logic and implementing it as a separate protocol, that stands on its own.
I'm glad you brought this up because I strongly believe in the potential to monetize attention in a decentralized, transparent way that's predicated on free market principles.
A protocol like this will open up so many ways to monetize and many different types of tokens will be born to cater for various use-cases and economic models.
I've got my plate full with Hive related projects at the moment and I'll post updates on my progress :)
Let’s talk. DM me on twitter or discord dhenz