I like the idea of standardizing how a community can make itself more sustainable.
A sustain.md file can be one standardized component. Yes, an RFC for that would be great because it invites a grand discussion, spreads the idea, invites much needed feedback, and helps revise and refine the standard.
On a larger picture, the sustain.md is only one best practices. It lacks a larger incentive system and there are other best practices. To separate the discussion around best practices, I started a thread on the idea of a Sustainable Best Practices Badge and wrote a blog post about it.