Done incorrectly, this can sound passive aggressive, but I suggest adding a short notice in your README:

- Feature requests should follow ((list your rules)). All requests that don't follow the rules will be ((closed|ignored))

- ((The maintainers of this project)) will only add new features ((at their leisure)). If you cannot contribute a pull request for ((feature X)), make a feature request ((following the rules on item 1)) and wait. Please do not be impatient.

- If you can implement ((feature X)), please do so and open a pull request.

My personal experience is that dealing with third-party code, merge conflicts and waiting for responses can be even more draining than writing code, so there's that.