Thanks! I have indeed been happy Streamlit-ing! 
Could be anything really. An example would be when clicking on the download link to the generated rules in this app, it will take you to a BTCpay or Square invoice page and continue to the download once the invoice has been paid 
I think something like this might be a good place to start since there wouldn’t have to be auth and tracking user accounts. So a basic version would just be a “pay to do this” button that generates an invoice.
And this could be used in creative ways I’m sure. Like different sections of apps which you get to with different buttons with different prices. Or even something like a buy button under images (though I imagine most will not use streamlit to buy/sell art, just an example xD).
And of course a subscription for tools will be very useful too and a perhaps a natural progression to the “pay to do this” button from the first example. I could imagine @jrieke’s app having a subscription for generating advanced templates with more granular settings.
Beyond this, depending on how things go, tiered pricing for hosting on better hardware could be useful as @robmarkcole mentioned.
Btw, speaking of auth. A little off topic but I think Telegram would be a great (and maybe simpler? 
) way to login to apps using a Streamlit bot