Here a mockup how it could look like on nodebb.org forum:
You may have noticed I now have a user icon instead of an avatar... we're testing something new because we're looking to remove gravatar from core
New users will still show their old avatars (gravatars or not), so hopefully this isn't a breaking change. Please report any oddities here (such as user icons missing).
These look awesome! I did something like that for BuddyPress (remove the need for Gravatar) by drawing GitHub-style "identicons" (see here) for users without a profile photo. My project used the GD PHP library so it won't be useful here but it's great to see you're going to be removing (many) calls to the (external) Gravatar service.