Use Argon2 or Bcrypt as password hash
Currently Pbkdf2 is in use. Is there a reason for this? Bcrypt has been widely tested for over 15 years with no known vulnerabilities and is in general the go to recommendation for a secure password hash.
Personally I like Argon2. It’s the winner of the Password Hashing Competition and is memory-hard, thus providing more protection against GPU cracking attacks. Because it’s not as tested and proven as Bcrypt or Pbkdf2, I understand, if there are some objections to it.
Comeonin, the library Pleroma uses for password hashing, supports all three algorithms.
If we would like to change the default password hash, an upgrade path has to be implemented, to convert the older hashes to the new algorithm.