First, I'd advise that we call these 'v2 fingerprints' so it's clear that we intend to substitute these anywhere traditional fingerprints are used.
Isn't using "fingerprint" not a bit misleading since it is not the output of a hash function but the ed25519 master public key itself?
Second, I would advise against truncated base64 identifiers. Fingerprints are 40 character hex. master-key-ed25519's base64 value can include slashes (such as "yp0fwtp4aa/VMyZJGz8vN7Km3zYet1YBZwqZEk1CwHI") which will be problematic for DirPort urls, GETINFO commands, etc.
The simplest solution would be to simply hexify these values. This will raise our fingerprint length from 40 to 64 characters
to avoid increasing the length to 64 characters, how about using urlsafe base64 that does not make use of the "/" character? https://tools.ietf.org/html/rfc4648#section-5 https://docs.python.org/3/library/base64.html#base64.urlsafe_b64encode