We welcome any kind of contribution -- contributors, code, users/testers, donations, or even just spreading of word. Both ways you suggested: rallying contributors, donating to the project or to other developers, are all greatly appreciated.
There are a number of issues outside the current "TB-focused" roadmap that we won't have time to take care of in the near-term; these are all posted as issues in this repo.
We usually encourage people who wish to contribute to prioritize their own use cases -- as they will be most familiar to their own context and needs. Short of asking you to go through the list of issues, if we have some context on how your community uses RNP, we might be able to point you to specific (pending) issues that you might be interest in.
As seen in https://github.com/rnpgp/rnp/issues/1119#issuecomment-639347305 we do not have an established mechanism for people who wish to contribute.
We should probably describe this in the website.
Some text that may be reused: