In the TechSpot Forums we have the default avatars set depending on their user id if they haven't set a custom avatar. If it ends with 0, 1, ... or 9 it shows a different avatar. Currently we made the changes directly to XenForo_Template_Helper_Core but I know this is not ideal, I want to create an add-on to rewrite the _getDefaultAvatarUrl if it's possible. Ideally being able to subclass XenForo_Template_Helper_Core would solve it. I'll try replacing the 'avatar' helper and copying some of the functions to my helpers class.