I see potential within the idea of a new category "Market" where users may offer money for new plugins with defined functionality.
So not money for the right to use a plugin, but a bonus motivation for the potential creators.
What I don't like is some sort of premium plugins you have to pay to use, but that's not my decision
Since NodeBB is open source I think this would rather be in the devs mind than sth. like "I've written a plugin, if you want to use it you have to pay for it".
A negative point is that there is no such system (yet) besides reputation to mark users as trustworthy, so I couldn't mark xyz as so. who doesn't pay after I made the requested plugin.
Maybe it'd be more nice to have a complete board about this instead of just a category. Then the reputation would be a valid measure of trust. In this case (once the feature is available) it could be an invite-only board, therefor it's not as easy to just create a new user once reputation is negative.