This Plugin replaces the get_avatar function in pluggable.php to pass an alternate image to gravatar.com that generates the user’s display_name if they do not yet have a gravatar.
More features coming soon…
Plugin Updated Nov-17th

Display Name If No Gravatar

This Plugin passes an alternate image to gravatar.com that generates the user's display_name if they do not yet have a gravatar.

nafsadh

Validate Profile Credentials and Names
In a WordPress site users have multiple names in their profile, the user login, display name and nick name. In a multi-user site, there is often a need to keep them unique. You need to allow users have only credible names, such that,
<ul>
<li>All display names are unique</li>
<li>All nicknames are unique</li>
<li>No nickname is a display name or login name of another user</li>
<li>No display name is a nickname or login name of another user</li>
</ul>
Credible Names is a easy plugin to help you with that.

Credible Names

Ensure user's nickname and display name are unique and not a display name or nick name or user name of someone else

Display Name If No Gravatar vs Credible Names