Nowadays people don't like to remember different passwords for different sites. Also saving passwords in a browser or any app is dangerous, passwords can be stolen by anyone (most probably hackers).

Features like "Login/Signup using One Time Password(OTP)" through email/SMS or both, which is valid for some limited time(5-15minutes) would be great.
An OTP is more secure than a static password, especially a user-created password, which can be weak and/or reused across multiple accounts. OTPs may replace authentication login information or may be used in addition to it to add another layer of security.

By creating a plugin for this feature, users don't have to remember or store their passwords.
Also, it would keep unauthorized users away from OsClass sites. This would also create a great user experience.

Many websites already started using this feature. Even there are lots of plugins available in WordPress.
A plugin like this in Osclass Marketplace will be sold much faster. You can also integrate the "SMS Notification and Verification Plugin" in this new plugin.

Eagerly waiting to see this plugin in the Marketplace.

You may use Two-factor authentication plugin to enhance security via OTP, but it is not replacement, but security enhancement.
What you propose eould still require user to use password for registration and auth app setup, looks like waste of time for user, its easier to remember password. Or?