What changes to OctoPrint would be necessary...?
I'm not completely convinced that any changes would be necessary. It would be nice, though, if some sort of tip jar mechanism were part of the infrastructure.
Not that this is really foosel's problem, but there could be an entirely different list of Premium plugins listed in the Plugin Manager if selected by the user.
If I thought that someone were willing to pay $50 each for the J.A.R.V.I.S. (Alexa) app support, I might consider making a general service for others. I know that sounds rich but it still wouldn't pay for the per-person support which it would force me to provide.
Pro's/con's: There was a time when people expected to pay for what they consumed. People now seem to want to freely consume without too much regard to what it costs the "generous vendor" in this case.
One could, though, just target features only a company could consume (printer farm Premium plugins). "Control 100 3D printers? Sure, that will be $500 $5000 for the basic package."
There are websites where one can—as a customer—ask coders to bid for a project. It would be interesting to allow people to describe what they want in a new plugin concept and plugin authors then charge an arbitrary stipend to start the work or charge a fee to deliver the plugin, provide support and work with the first user to make sure it meets their needs.