I thought about this too but I can see how many steemians may not want an outside advertiser network on steemit. You can already use affiliate links and banners in your posts to make extra money after 7 days.
I can see 2 approaches to this:

1. Develop a full advertising exchange platform that runs on steem but work similiar to adsence where only payment would be in steem. These ads could also be displayed outside of steemit on personal sites but still get paid steem.
2. Partner with an existing network that's fairly big and work with them to implement a steem payment system that would function like option 1.