Proposition
Individual password fields are introduced when importing a new keystore. This would allow for custom keystore passwords with -keypass and -storepass.
Reason
As per Android Studio for publishing,
If you choose to manage and secure your app signing key and keystore yourself (instead of opting in to Play App Signing), securing your app signing key is of critical importance, both to you and to the user. If you allow someone to use your key, or if you leave your keystore and passwords in an unsecured location such that a third-party could find and use them, your authoring identity and the trust of the user are compromised.
If a third party should manage to take your app signing key without your knowledge or permission, that person could sign and distribute apps that maliciously replace your authentic apps or corrupt them. Such a person could also sign and distribute apps under your identity that attack other apps or the system itself, or corrupt or steal user data.