I have been thinking of an in-house Ad service for steemit for the past few days. I think it's a really good idea if it can serve to benefit steemit and it's members as a whole, much like you said in your post.
I really like the idea of Revenue Sharing amongst the members a lot. Keeping the ads circular and contained only within steemit is a great idea too I'd think. Of course it could be circumvented with people posting things related to stuff not steemit related and then enticing them to follow the trail elsewhere... also known as "bridge marketing" but the fact that this is a self regulating platform I don't think anything that wasn't carefully crafted and considered would last very long.
I'm sure ideas would have to be fleshed out but it's a very interesting idea and it seems it would almost be self perpetuating for those who hold enough steem power or dollar or whatever would be used. This could cause an issue so that would have to be factored in too.