Elgg currently provide various means of authentication which is sufficient for most current needs.  However, to help further distinguish Elgg, a feature that will allow ALL Elgg public facing sites to be networked, with permission from the admins but only for specific/approved sites will place Elgg up a notch as I don't think there is anything out there that does that.

Think of this much like an Elgg site as a member.  A site can then "friend" another site (one-way trust relationship) and the friend site admin will get a notification which they can either ignore or reciprocate.  The river can then flow within these sites, either one way or both ways with the option to allow users to access the friend site from their home site seamlessly.

Thoughts?