Secure password reset
You probably don't want to encrypt data with a key derived from a password directly with a KDF. This makes changing the password require rencrypting everything. You can instead encrypt data with a strong random key. And encrypt the key with a password derived key. This encrypted key is what you keep.
You can also encrypt this key with other keys. Including a random key the user downloads and keeps wherever, possibly on paper. Or you can possibly split a key into multiple shares to be kept separately. 

Obviously if you have multiple ways to decrypt your key, it is only as secure as the weakest method.