Module for username suggestions on registration
There is no module that does that. The closest you will find is the one that just checks if the username is available while registering. An example of such module is username check. To achieve the username suggestion bit, you may have to write a custom module for that.


  This very simple module allows visitors to check username originality
  quickly using AJAX request during registration (completing
  registration form). This makes registration process more convenient as
  you don't need to submit your registration form to find out whether
  this username is already taken or not. This module is sensible to
  standard Drupal username validation as well as Access Rules
  validation.


or friendly register


  Friendly Register module allows users to see if a username or email
  address has already been used during registration before they submit
  the form. This module checks the database and returns an error if the
  username is already in use. In addition to checking the username the
  module checks if there is already an account using that email address,
  if there is, a message is displayed with links to the login or reset
  password pages.