Any non-C library? Rust? Python? Any other ways to avoid C?
I wouldn’t worry about GnuPG. It will always be possible to sign with multiple tools at once. Well, obviously just don’t use file extensions that are already taken by others such as .asc, .sig, .gpg.
I wouldn’t worry much about GnuPG compatibility. I’d suggest to think big and design this as a tool that can be used to replace GnuPG. Similar how signify was implemented. Legacy free.
But then the digest files might require version numbers because how else at a later time the program would now to insist on which algorithms to verify.
I don’t think this is how GnuPG development works. That has an almost zero percent chance of working. Check this out, dunno about it:
If you want to land patches in GnuPG or others you need to discuss this with them beforehand. Otherwise might hit a blocker.
Sequoia isn’t a a fork of gpg. It’s a rewrite from scratch in Rust. It implements the OpenPGP standard.
Both, GnuPG and Sequoia might be adamant about standardization.