A new username must be in the form of:

A new username must be in the form of an email address because this format is widely recognized and ensures a unique identification for users across various platforms. Using an email address as a username aligns with established practices in many online services, facilitating account creation and recovery processes. It also provides a straightforward method for users to remember their credentials, as they typically use their email for communication and other online activities.

In contrast, a phone number is not a standard username format, as it can vary in length and structure across different regions. A username of any length is too vague; there are often specific character limits and formatting rules that must be adhered to. Lastly, social media handles, while unique, do not generally serve as usernames outside of their originating platforms and often come with specific parameters that may not translate well to other systems.