Short answer
My idea would be for paid extension developers to be "official" from Joplin. That is, you to advertise your plugin on the Joplin site must be certified. If you are not certified, you cannot disclose your proprietary extensions or themes.
The idea of a paid advertisement would be for the plugin to appear on the official page of the Joplin site. Because for me it would make sense if I want to have more users, to pay an ad to view my paid extensions for more users.
In short, my idea is paid advertising and paid certification. That is, they are complementary ideas, but they do not depend on each other.
Long answer
This might work, for example, for a business marketplace, where companies want plugins to solve certain things like exporting pdfs, synchronizing data with some server, etc. Also, as mentioned earlier this could help with the idea of commercial plugins for end users or companies for certain things like the synchronization I mentioned or specific features.
In addition, any end user and company that adopts free software wants the same software to be more integrative, offering different options, themes and plugins. In this sense, having the support of developers, users and an active community is the crucial point in choose. In theory this allows Joplin to possibly be more adopted as it will have different types of developers offering different types of plugins, themes and services in Joplin if this is of interest to the Joplin community as well.
Evernote, for example, adopts the consulting model, where it hires people to solve end-user or companies problems. The idea of Joplin certification came from this idea, but instead of being a simple consultancy, it could offer some possibilities if the person is a developer. For example, if the person is a developer, they can publish themes and plugins on the Joplin website. If the person does not work with development, they can simply offer courses, training given the certificate validation period.
Maybe this certification validation period could be 1 year or 2 years, it would be a way for the community to have money to invest in new open projects. Maybe something like 100 bucks, I don't know, the community would have to see if this idea is good or not.
In the case of Evernote consulting, there are some restrictions imposed, I'm not sure what the restriction would be but there would be some restriction, maybe you as an Evernote consultant can't work as a consultant in a company that competes with Evernote. Anyway,
it would be a little different from this consulting idea of Evernote. Maybe... a good example is a LibreOffice consultancy is something like what I say here.
A big difference would be this: if you are not Joplin certified, you would have to pay to advertise proprietary plugins. If you are Joplin certified, you will receive a discount on proprietary plugin ads. And it would have greater visibility for end users and companies using Joplin Cloud.
My main idea with paid certification would be to bring more trust and branding to the Joplin community. In conjunction with a commercial plugin idea I commented on earlier. I'm not saying the community or the Joplin software is bad, I'm just looking for ways to make Joplin more adopted and used. So... If this is good or bad I want to know everyone's opinion here.
Sorry sorry for the big text.