An ad network would be also a good idea. But I think this would be very difficult to realize in an entirely decentralized manner, because a blockchain-based service very probably could never reach the speed and customization of an centralized ad server network like Doubleclick . So a steem-based ad service would probably work more like Anonymous Ads or CoinURL today, with a centralized entity providing the ads. Such a service could be setup relatively easy by everyone with some knowledge about web hosting.