I would advise against keeping content locked under a proprietary license.
There's a few reasons to provide a libre/free license (like CC-BY-SA):
  • It helps guard against future irrelevancy by making the data easy to replicate
  • It provides a potentially rich dataset for programmers, entrepreneurs etc.
  • In general, it gives back to the community and is much in the spirit of what Bitcoin is about, libre/free source that allows for re-use and modification, even for commercial reuse
Were you to not put the content under a libre/free license, you're heavily favoring you or your organization as gatekeeper to the knowledge. In this case, SN would most likely be effectively be the sole licensee of the data which means everyone in this community is now not only giving you sats to participate but also handing you effectively sole use of their intellectual property.
It looks like you have a potential business model in mind in terms of "paying for articles" (like Y'alls?). From my perspective, the fundamental question is what's actually gained by providing content that isn't libre/free for the community to use.
Your concern is that someone won't pay if the content is under a libre/free license? Under what scenario would you be using the intellectual property angle to enforce copyright violations? Notice on HN that the first post to paywalled articles is a link to a non-paywalled archive site so people can read the articles.
My view is that convenience supersedes a lot of these concerns. People don't care about spending a couple sats, especially considering we're doing it anyway by tipping or upvoting, so providing easy avenues to pay for articles is the main issue, not whether we can view the article by paying the absolute minimum for it.
In terms of providing options for individuals to change the license of their content, I would also advise against it. It's confusing and any re-use of the dataset now becomes problematic to use. At best, users direct content needs to be filtered. At worst, anyone using the dataset now has to do a similarity test for all the content against the proprietary portions to make sure they're not in violation of the users license choice.
My view is it's reasonable to ask participants to put their content under a CC-BY-SA license. The content is on the internet, it's pretty much available anyway. Instead of shying away from it, lean into it and provide it under a libre/free license. The price to participate in the community is that you're adding the commons by doing so. The license is just being explicit about it.
On a personal note, one of the reasons why I'm excited about this community is because it has the possibility to embody portions of the ideology behind Bitcoin I find compelling. I know if there were a system that funneled the intellectual property to SN for essentially exclusive use, I would be less interested in participating and championing it.

In terms of the API, the first step is to make the data available to people to read. I imagine the primary use case is to query or download the dataset for some use. Providing an easy facility to do that, maybe by providing better documentation, examples, etc. is the primary concern.
If people want programmatic access to their accounts through the API, that's cool, but not a feature you need right now?
Your concern is that someone won't pay if the content is under a libre/free license? Under what scenario would you be using the intellectual property angle to enforce copyright violations? Notice on HN that the first post to paywalled articles is a link to a non-paywalled archive site so people can read the articles.
No, my concern is users won't post certain kinds of content, particularly valuable original content, if in doing so they permit anyone else to use it however they'd like.
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Alright, well, regardless, the IP will still be licensed by SN and SN will be positioned to be the gatekeeper to that knowledge.
I think this is standard startup playbook stuff, whereby a platform is created, a moat is built around all the content and everyone is funneled into a walled silo of content.
When I look at Twitter, Facebook and the rest of social media, the only ray of hope is cryptocurrency and some decentralized content, where incentives are given by the currency and decentralization is bolstered by the libre/free license.
In my opinion, sites that don't actively try to make their content accessible are doomed to have their content disappear. I don't know if Experts Exchange has their knowledge base available in any meaningful way but they've faded to irrelevancy. AOL has long since faded. My belief is that it's only a matter of time before Twitter, FB, Quora and even HN have their data lost. They're never incentivized to make the data available to the community and if they ever do, because they're failing, it's often too late as they're already passed the threshold of irrelevancy.
StackOverflow, Wikipedia etc. have embraced the libre/free idea and, in my opinion, are better for it. When I think of sites like this, I envision some hybrid of SO and Wikipedia where the digital currency allows for direct monetary rewards to contributors.
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