I really like that idea, it wouldn't work for a large portion of the content here, and a large portion of my own content, but certain tags and subjects that would be such an amazing idea. Fiction in particular, stories and poetry etc., would work so well with that. I've just had a quick look, and Amazon offer a Print-in-demand service, I wonder if you could even get them actually published in a physical format too?

I've read other users bring up the issue of content that's over 7 days old doesn't really get noticed anymore, once payout has been received it's like the posts are just abandoned with no real way to search for them in the Steemit interface, other than scrolling through a users posts.

I was thinking a series subscription more like a RSS-feed, I don't agree with paywalls, basically exactly what you've described with series codes. To be honest, the larger platforms like YouTube haven't even quite perfected that feature yet either, but with Ginabot I think all the functionality is probably already there to use.