The ethos and intent of Firn is still developing. Contributions are welcome, but until a more readily-available roadmap exists, you may wish to consider opening an issue or to ask questions in the Discord channel before starting a large contribution/patch.
With that said, these things come to my mind when I am building Firn.
- Cover the first 75% of the more popular features of org-mode, and cover more niche features later.
- Stay close to rendering plaintext/html. Avoid JavaScript and Frontend bloat.
- Make it extensible with a small core.
- Be kind.
- Feature requests will be considered but there is 0% guarantee it will be integrated into the project.